Support GEVENT_RESOLVER_SERVERS for configuring dnspython nameservers.

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

Fixes #1103.

ccache saves us time on compiling, but not linking. The difference is unlikely to be significant for test purposes.

Make socket.sendall() fast on PyPy3: 60MB/s -> 600MB/s

I see in on OS X even without gevent involved. I see it with gevent involved both on the sendall branch and the master branch. I see it on Travis (now) with the sendall patch.
I don't know if it's been seen before the sendall patch on Travis---flaky builds tend to get rerun. None of the other SSL tests are failing, though, and the pypy version of test_ssl.py notes that test_handshake has been removed from CPython altogether.
It looks like it's very dependent on buffering and things like that.

TestEtcHosts round-robin failures also seen on appveyor with the system resolver---because we're not using the local system hosts file.

Share the chunking code between Python 2 and 3.

Also get the tests passing (read: skip a bunch) with 5.10.1 on OS X,
which is apparently less well tested than Linux.

We don't test pypy3 on appveyor, so:

[skip appveyor]

Simplify libev default loop handling and let it regenerate itself.

Don't let destroying the default C loop from two different Python loo…

Don't let destroying the default C loop from two different Python loop objects crash the process. Fixes #1098

Take a whack at using ccache to speed up builds on CI.

[skip ci]

Centralize configuration in a ``gevent.config`` object.

Extra tests for _config. Fix a long-standing bug where package paths in import names did not clean up after themselves.

Before I realized that was the root cause, I refactored to avoid
starting the hub greenlet *at all*, and that also fixed the problem.

The problem was that some versions of CPython using the libev-cext
loop (and only that one) would crash garbage collecting a
greenlet (presumably the hub greenlet, which was attached to a
destroyed loop):

0   libsystem_kernel.dylib        	0x00007fff66455e3e __pthread_kill + 10
1   libsystem_pthread.dylib       	0x00007fff66594150 pthread_kill + 333
2   libsystem_c.dylib             	0x00007fff663648fe raise + 26
3   libsystem_platform.dylib      	0x00007fff66587f5a _sigtramp + 26
4   ???                           	000000000000000000 0 + 0
5   greenlet.cpython-35m-darwin.so	0x000000010d76b297 green_clear + 183
6   org.python.python             	0x000000010c707e0c collect + 1824
7   org.python.python             	0x000000010c7076e3 _PyGC_CollectNoFail + 43
8   org.python.python             	0x000000010c6e81d4 PyImport_Cleanup + 968
9   org.python.python             	0x000000010c6f1b49 Py_Finalize + 92
10  org.python.python             	0x000000010c6f20d5 Py_Exit + 13
11  org.python.python             	0x000000010c6f4631 handle_system_exit + 320
12  org.python.python             	0x000000010c6f42a1 PyErr_PrintEx + 42

We should take a look at that and see if we can do something more graceful.

The gevent.select.poll object must keep a tighter handle on the lifetime of io watchers it spawns. Previously they were leaking if not unregistered, which some callers fail to do --- looking at you, dnspython.

This lets us document things more clearly, and, with care, allows
configuring gevent from Python code without any environment varibles.

All the fileobject implementations needed to be moved out of the
``gevent.fileobject`` module so as not to introduce import cycles that
made configuring from code impossible.

Fixes #1090

Add support for /etc/hosts to dnspython resolver.

Based on code from eventlet, but heavily refactored and modified:

- Split host parsing into its own class, re-used from the test case
- Hosts handle PTR lookups for ipv4
- Some bug fixes around what should be strings where.
- Allow strings for the rdtype paramater
- Watch last modified time instead of reloading the file at fixed
  intervals.

More of our DNS tests pass now (notable TestEtcHosts is now enabled
for this resolver), and we tend to generate more conformant results
than the ares resolver (by observation, not tested).

Let dnspython resolver be used without monkey-patching