Remove Gevent-specific code

PyMongo 2.x has Gevent-specific code to support Gevent with or without patch_thread. However, patch_socket is always required with Gevent.

In PyMongo 3, remove all Gevent-specific code and rely on Gevent's patch_all. Gevent support in PyMongo 3 will be equal to support for Eventlet, Greenhouse, and any future monkey-patching greenlet frameworks.

Remove the "use_greenlets" option.

Review PyMongo's testing under Gevent patch_all to ensure continued coverage.