Fixes #1118.

Also some speed improvements for semaphores.

Speed up gevent.local.local more, especially for subclasses (2-3x faster)

Do this with some cython tricks and some caching of type attributes.
Cython 0.28 compiles this code to be quite a bit faster than Cython
0.27 does (the below uses 0.28), but it's still a win on both.

The type caching could potentially be a compatibility issue, but in
practice I suspect it won't be.

Benchmarks on 3.6:

+--------------------+------------------+-----------------------------+
| Benchmark          | 3.6              | 3.6 This Branch             |
+====================+==================+=============================+
| getattr gevent     | 190 ns           | 158 ns: 1.20x faster (-17%) |
+--------------------+------------------+-----------------------------+
| setattr gevent     | 180 ns           | 165 ns: 1.09x faster (-8%)  |
+--------------------+------------------+-----------------------------+
| getattr gevent sub | 540 ns           | 175 ns: 3.09x faster (-68%) |
+--------------------+------------------+-----------------------------+
| setattr gevent sub | 528 ns           | 179 ns: 2.95x faster (-66%) |
+--------------------+------------------+-----------------------------+
| setattr native     | 80.8 ns          | 78.8 ns: 1.03x faster (-2%) |
+--------------------+------------------+-----------------------------+

Not significant (3): getattr native; getattr native sub; setattr native sub

Benchmarks on 2.7:

+--------------------+------------------+-----------------------------+
| Benchmark          | local_27_master2 | local_27_tweak2             |
+====================+==================+=============================+
| getattr gevent     | 162 ns           | 158 ns: 1.03x faster (-3%)  |
+--------------------+------------------+-----------------------------+
| setattr gevent     | 173 ns           | 165 ns: 1.04x faster (-4%)  |
+--------------------+------------------+-----------------------------+
| getattr gevent sub | 471 ns           | 181 ns: 2.61x faster (-62%) |
+--------------------+------------------+-----------------------------+
| setattr gevent sub | 462 ns           | 190 ns: 2.44x faster (-59%) |
+--------------------+------------------+-----------------------------+
| getattr native     | 87.4 ns          | 89.7 ns: 1.03x slower (+3%) |
+--------------------+------------------+-----------------------------+
| setattr native     | 133 ns           | 110 ns: 1.20x faster (-17%) |
+--------------------+------------------+-----------------------------+
| getattr native sub | 101 ns           | 97.9 ns: 1.03x faster (-3%) |
+--------------------+------------------+-----------------------------+
| setattr native sub | 118 ns           | 111 ns: 1.06x faster (-6%)  |
+--------------------+------------------+-----------------------------+

PyPy unfortunately shows as a little slower, though I'm not sure I
fully believe that:

+--------------------+-------------------+-----------------------------+
| Benchmark          | local_pypy_master | local_pypy_tweak2           |
+====================+===================+=============================+
| getattr gevent     | 135 ns            | 157 ns: 1.16x slower (+16%) |
+--------------------+-------------------+-----------------------------+
| setattr gevent     | 126 ns            | 162 ns: 1.28x slower (+28%) |
+--------------------+-------------------+-----------------------------+
| getattr gevent sub | 150 ns            | 175 ns: 1.17x slower (+17%) |
+--------------------+-------------------+-----------------------------+
| setattr gevent sub | 153 ns            | 183 ns: 1.20x slower (+20%) |
+--------------------+-------------------+-----------------------------+
| getattr native     | 0.18 ns           | 0.19 ns: 1.05x slower (+5%) |
+--------------------+-------------------+-----------------------------+
| setattr native     | 0.18 ns           | 0.20 ns: 1.06x slower (+6%) |
+--------------------+-------------------+-----------------------------+
| getattr native sub | 0.18 ns           | 0.19 ns: 1.05x slower (+5%) |
+--------------------+-------------------+-----------------------------+
| setattr native sub | 0.19 ns           | 0.18 ns: 1.06x faster (-6%) |
+--------------------+-------------------+-----------------------------+

[skip ci]
Current numbers:

local_36_master
===============

getattr gevent: Mean +- std dev: 216 ns +- 6 ns
setattr gevent: Mean +- std dev: 215 ns +- 6 ns
getattr gevent sub: Mean +- std dev: 600 ns +- 19 ns
setattr gevent sub: Mean +- std dev: 591 ns +- 38 ns
getattr native: Mean +- std dev: 56.9 ns +- 4.1 ns
setattr native: Mean +- std dev: 84.3 ns +- 4.9 ns
getattr native sub: Mean +- std dev: 65.6 ns +- 5.3 ns
setattr native sub: Mean +- std dev: 76.2 ns +- 4.6 ns

local_27_master
===============

getattr gevent: Mean +- std dev: 177 ns +- 7 ns
setattr gevent: Mean +- std dev: 192 ns +- 6 ns
getattr gevent sub: Mean +- std dev: 501 ns +- 10 ns
setattr gevent sub: Mean +- std dev: 536 ns +- 25 ns
getattr native: Mean +- std dev: 93.8 ns +- 3.6 ns
setattr native: Mean +- std dev: 118 ns +- 6 ns
getattr native sub: Mean +- std dev: 102 ns +- 4 ns
setattr native sub: Mean +- std dev: 118 ns +- 8 ns

local_pypy_master
=================

getattr gevent: Mean +- std dev: 135 ns +- 6 ns
setattr gevent: Mean +- std dev: 126 ns +- 4 ns
getattr gevent sub: Mean +- std dev: 150 ns +- 4 ns
setattr gevent sub: Mean +- std dev: 153 ns +- 7 ns
getattr native: Mean +- std dev: 0.18 ns +- 0.00 ns
setattr native: Mean +- std dev: 0.18 ns +- 0.01 ns
getattr native sub: Mean +- std dev: 0.18 ns +- 0.01 ns
setattr native sub: Mean +- std dev: 0.19 ns +- 0.01 ns

Add Greenlet.minimal_ident, a small, sequential, reusable counter for greenlets

Fixes #755

Add spawn_tree_locals, spawning_greenlet and spawning_stack to gevent.greenlet.Greenlet

Small improvements to creating the objects, somewhat larger
improvements to joining.

From bench_spawn on 3.6.4 before any work:

//gevent36/bin/python ./bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 8.71 microseconds per greenlet
sleep(0): 16.99 microseconds per greenlet
 joining: 7.34 microseconds per greenlet

//gevent36/bin/python ./bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 10.25 microseconds per greenlet
sleep(0): 18.89 microseconds per greenlet
 joining: 7.14 microseconds per greenlet

And now:

//gevent36/bin/python ./bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 11.56 microseconds per greenlet
sleep(0): 16.76 microseconds per greenlet
 joining: 5.74 microseconds per greenlet

//gevent36/bin/python ./bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 11.49 microseconds per greenlet
sleep(0): 17.16 microseconds per greenlet
 joining: 5.31 microseconds per greenlet

We're now only ~2x slower, instead of 10x.

Based on #755.

A comment in the code goes into detail about the timing. Here it is
again:

 Timings taken Feb 21 2018 prior to integration of #755
 python -m perf timeit -s 'import gevent' 'gevent.Greenlet()'
 3.6.4       : Mean +- std dev: 1.08 us +- 0.05 us
 2.7.14      : Mean +- std dev: 1.44 us +- 0.06 us
 PyPy2 5.10.0: Mean +- std dev: 2.14 ns +- 0.08 ns

 After the integration of spawning_stack, spawning_greenlet,
 and spawn_tree_locals on that same date:
 3.6.4       : Mean +- std dev: 8.92 us +- 0.36 us ->  8.2x
 2.7.14      : Mean +- std dev: 14.8 us +- 0.5 us  -> 10.2x
 PyPy2 5.10.0: Mean +- std dev: 3.24 us +- 0.17 us ->  1.5x

Selected bench_spawn output on 3.6.4 before:

//gevent36/bin/python src/greentest/bench_spawn.py eventlet --ignore-import-errors
using eventlet from //gevent36/lib/python3.6/site-packages/eventlet/__init__.py
spawning: 11.93 microseconds per greenlet
sleep(0): 23.49 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 3.39 microseconds per greenlet
sleep(0): 17.59 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 8.71 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 2.09 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py none --ignore-import-errors
    noop: 0.33 microseconds per greenlet

And after:

//gevent36/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 12.99 microseconds per greenlet -> 3.8x

//gevent36/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 19.49 microseconds per greenlet -> 2.2x

//gevent36/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 4.57 microseconds per greenlet -> 2.2x

We're approximately the speed of eventlet now.

Refs #755

Current numbers on my workstation:

CPython 3.6.4

//gevent36/bin/python bench_spawn.py eventlet --ignore-import-errors
using eventlet from //python3.6/site-packages/eventlet/__init__.py
spawning: 11.93 microseconds per greenlet
sleep(0): 23.49 microseconds per greenlet

//gevent36/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 3.39 microseconds per greenlet
sleep(0): 17.59 microseconds per greenlet

//gevent36/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 8.71 microseconds per greenlet
sleep(0): 16.99 microseconds per greenlet
 joining: 7.34 microseconds per greenlet

//gevent36/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 2.09 microseconds per greenlet
sleep(0): 13.57 microseconds per greenlet

//gevent36/bin/python bench_spawn.py none --ignore-import-errors
    noop: 0.33 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs eventlet --ignore-import-errors
using eventlet from //python3.6/site-packages/eventlet/__init__.py
spawning: 12.06 microseconds per greenlet
sleep(0): 24.25 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 4.62 microseconds per greenlet
sleep(0): 22.18 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 10.25 microseconds per greenlet
sleep(0): 18.89 microseconds per greenlet
 joining: 7.14 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 3.27 microseconds per greenlet
sleep(0): 15.03 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs none --ignore-import-errors
    noop: 0.50 microseconds per greenlet

CPython 2.7.14

//gevent27/bin/python bench_spawn.py eventlet --ignore-import-errors
using eventlet from //python2.7/site-packages/eventlet/__init__.pyc
spawning: 17.84 microseconds per greenlet
sleep(0): 31.42 microseconds per greenlet

//gevent27/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 4.95 microseconds per greenlet
sleep(0): 21.40 microseconds per greenlet

//gevent27/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 12.44 microseconds per greenlet
sleep(0): 22.90 microseconds per greenlet
 joining: 10.68 microseconds per greenlet

//gevent27/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 2.33 microseconds per greenlet
sleep(0): 15.11 microseconds per greenlet

//gevent27/bin/python bench_spawn.py none --ignore-import-errors
    noop: 0.58 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs eventlet --ignore-import-errors
using eventlet from //python2.7/site-packages/eventlet/__init__.pyc
spawning: 19.38 microseconds per greenlet
sleep(0): 31.62 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs gevent --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 6.59 microseconds per greenlet
sleep(0): 23.12 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 13.78 microseconds per greenlet
sleep(0): 21.65 microseconds per greenlet
 joining: 10.86 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 3.59 microseconds per greenlet
sleep(0): 16.14 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs none --ignore-import-errors
    noop: 0.65 microseconds per greenlet

PyPy2 5.10.0

//geventpypy/bin/python bench_spawn.py eventlet --ignore-import-errors
using eventlet from //geventpypy/site-packages/eventlet/__init__.pyc
spawning: 29.91 microseconds per greenlet
sleep(0): 84.24 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 0.73 microseconds per greenlet
sleep(0): 33.02 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 2.49 microseconds per greenlet
sleep(0): 38.56 microseconds per greenlet
 joining: 13.08 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 0.52 microseconds per greenlet
sleep(0): 29.63 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py none --ignore-import-errors
    noop: 0.02 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs eventlet --ignore-import-errors
using eventlet from //geventpypy/site-packages/eventlet/__init__.pyc
spawning: 30.36 microseconds per greenlet
sleep(0): 83.00 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs gevent --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 1.56 microseconds per greenlet
sleep(0): 34.80 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 3.48 microseconds per greenlet
sleep(0): 39.43 microseconds per greenlet
 joining: 16.21 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 1.51 microseconds per greenlet
sleep(0): 52.19 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs none --ignore-import-errors
    noop: 0.13 microseconds per greenlet

[skip ci]

Make the check_callback do stuff under PyPy to trigger signal deliver…

Cleanup the libev destroyed data like the libuv destroyed data.

Fix race conditions in libuv child watchers.

(hopefully) fix a crash destroying loops in test__core.py; only seen on Windows, but with some tweaks I could reproduce it locally.

Fixes #1104

Be more careful about issuing the SSL warning on Py2.

Also some other doc updates.

[skip ci]

Fixes #1108.

Update c-ares to 1.14. Fixes #1105.

Support GEVENT_RESOLVER_SERVERS for configuring dnspython nameservers.

Also allow this for ares, and deprecate the GEVENTARES_SERVERS environment variable.

Fixes #1103.