Compared to the previous commit:

| Benchmark          | 36_threadpool_opt_PR | 36_threadpool_opt_cond10    |
+--------------------+----------------------+-----------------------------+
| imap_unordered_seq | 1.06 ms              | 1.02 ms: 1.04x faster (-4%) |
| imap_unordered_par | 965 us               | 928 us: 1.04x faster (-4%)  |
| imap_seq           | 1.08 ms              | 1.03 ms: 1.04x faster (-4%) |
| map_seq            | 785 us               | 870 us: 1.11x slower (+11%) |
| map_par            | 656 us               | 675 us: 1.03x slower (+3%)  |
| apply              | 1.14 ms              | 1.12 ms: 1.02x faster (-2%) |

Especially for map. None of the pools really need map to go through
imap since they have to wait for everything anyway and they return
results ordererd.

| Benchmark          | 36_threadpool_master | 36_threadpool_opt_cond5     |
|--------------------|----------------------|-----------------------------|
| imap_unordered_seq | 1.15 ms              | 1.07 ms: 1.08x faster (-7%) |
| imap_unordered_par | 1.02 ms              | 950 us: 1.08x faster (-7%)  |
| imap_seq           | 1.17 ms              | 1.10 ms: 1.06x faster (-6%) |
| imap_par           | 1.07 ms              | 1000 us: 1.07x faster (-7%) |
| map_seq            | 1.16 ms              | 724 us: 1.60x faster (-37%) |
| map_par            | 1.07 ms              | 646 us: 1.66x faster (-40%) |
| apply              | 1.22 ms              | 1.14 ms: 1.07x faster (-7%) |
| spawn              | 1.21 ms              | 1.13 ms: 1.07x faster (-7%) |

Add monitoring for memory usage, emitting events as it moves around the threshold.

Needs specific tests.

Start having the monitor thread emit events for monitored conditions.

This is the only reasonable path I could think of to enable memory monitoring, with gevent just being responsible for monitoring the memory and detecting overage conditions, while users plug in their own policies.

I wasn't even using them to debug anymore, they were too verbose to be blanket enabled.

Fixes #1146.

Also tweak the leakcheck tests for threadpools.

Fix libuv multiplex io watchers from polling too much when one event …

Fix libuv multiplex io watchers from polling too much when one event has been completely turned off. Fixes #1144.

Add support for a background monitoring thread to be associated with each hub.

And give it unittest coverage. (Using it as an actual thread didn't
produce coverage metrics because we use 'greenlet' concurrency.)

Add a rudimentary scheduler so that functions that don't want to run
as often as the minimum stay (very roughly) on their period.

The threadpolo worker greenlet itself is meant to spend much of its time blocking, so don't report on it if it does.

Only if we switch greenlets while running the user's code should we start looking for blocking.

Right now, it is used to detect blocked event loops, and it is extensible by users. In the future there will be some more default monitoring options (e.g., memory).

Refs #1021.

Make more performance-sensitive places use _get_hub_noargs, since under Python 3 it is twice as fast to call as get_hub.

Introduce GEVENT_TRACK_GREENLET_TREE to disable greenlet tree features

As a performance optimization for applications where spawning
greenlets is critical. Plus some other optimizations to speed up
spawning in the general case.

CPython 3.6 with 1.2.2 vs these changes with tracking disabled:

| Benchmark              | 36_122_bench_spawn | 36config_bench_spawn_tree_off |
+------------------------+--------------------+-------------------------------+
| eventlet spawn         | 12.6 us            | 12.2 us: 1.04x faster (-4%)   |
| eventlet sleep         | 5.22 us            | 4.97 us: 1.05x faster (-5%)   |
| gevent spawn           | 4.27 us            | 5.06 us: 1.19x slower (+19%)  |
| gevent sleep           | 2.63 us            | 1.25 us: 2.11x faster (-53%)  |
| geventpool spawn       | 9.00 us            | 8.31 us: 1.08x faster (-8%)   |
| geventpool sleep       | 4.82 us            | 2.83 us: 1.70x faster (-41%)  |
| geventraw spawn        | 2.51 us            | 2.81 us: 1.12x slower (+12%)  |
| geventraw sleep        | 649 ns             | 679 ns: 1.05x slower (+5%)    |
| geventpool join        | 3.47 us            | 1.42 us: 2.44x faster (-59%)  |
| geventpool spawn kwarg | 11.0 us            | 8.95 us: 1.23x faster (-19%)  |
| geventraw spawn kwarg  | 3.87 us            | 4.20 us: 1.08x slower (+8%)   |

The differences compared to master are hard to quantify because the
standard deviation ends up being more than 10% of the mean in many
cases---and about a 10% improvement is what we typically see, so it
goes back and forth.

Also monkey-patch threading._Event on Python 2. Fixes #1136

Use strongly typed watcher callback functions in libuv, preventing compiler warnings

Turns out we weren't passing the macros we thought we were at all. Whoops.

And also ensuring that in the Python code we know the type of pointer we're dealing with. This might catch some errors.

Fix documentation for Pool.full()

The original docs had the purpose of the boolean return value reversed. I fixed that distinction, and also tried to clarify how the method can be used.

Exclude 3.3, account for cython being a develop wheel right now.

[skip ci]