Fixes #1569

Properly clean up native resources in gevent.signal_handler() 

Fixes #1606

Also fix reporting errors from threadpool tasks, and make waiting for threadpool tasks a bit cheaper.

Let the greenlets be freed when destroying a hub

TypeError: initializer for ctype 'struct uv_loop_s *' must be a cdata pointer, not NoneType

Now it is safe.

join() still allows the hub to be re-entered, so we have to move the cleanup to destroy()

Addressing #1601.

This solution works, but breaks anything that tries to use the hub again after joining it once, because the hub greenlet dies. So that can't be right.

Fix compilation with Cython 3

This is because the memory leak tests failed: https://travis-ci.org/github/gevent/gevent/jobs/683782800

The double-dunder name changes broke some things.

Fixes #1599

Avoid closing the same libuv watcher object twice

Under some circumstances (only seen on Windows) that could lead the program to crash.

Update libuv to 1.36

Fixes #1597

Better exception handling for alternate resolvers

A few cases of gethostbyaddr don't match the exception.

Share quite a bit more code between the ares and dnspython resolver.

Addresses #1459

The c-ares resolver comes even closer to the system resolver.

Tests pass locally on a manylinux docker image. So take it out of the known flaky list for now, see how that does.

Use `ares_getaddrinfo`

Does not yet handle canonical names, but this doesn't result in test failures.