extended.py: Adds IOStream methods returning a queue for streaming.
http.py: Refactors Connection to process a queue for streaming calls.
http.py: Implements connection pool cleaning.
http: Adds unit tests to test TCP reset.

Does not reproduce issue at chfoo/wpull#27 :rage:.
http.py: Sets the close callback again if already connected (chfoo/wpull#27).

IOStream is wrapping the context. Maybe the context is leaked and that's
the problem?
changelog: Adds entry and updates latest to 0.17.2.
Merge branch 'develop'
Bumps version to 0.17.2.