Add simple concurrency support to HTTP20Connection

- adds simple scheme for allowing HTTP20Connections to be used
concurrently by adding RLocks.

- connection and stream creation is synchronized
- reads and writes of frames on the connection and its streams are
  synchronized
- connection#request is synchronized

The current tox tests pass, but no new unit tests are added.
However a threading test is added to test_release.py that passes, e.g

$ py.test -s \
test_release.py::TestHyperActuallyWorks::test_threaded_abusing_nghttp2_org

Author: Tim Emiola