keepalive: implement _close_conn() so closes are known
Keepalives were not working on Python 3 because
http.client.HTTPResponse was refactored to call _close_conn()
instead of close(). Our custom close() is what returns inactive
connections to the available state.
We better support Python 3 by implementing a _close_conn().
Differential Revision: https://phab.mercurial-scm.org/D5720