PYCBC-172: Implement Connection._close()

This can be used to forcibly free the internal system resources uses by
the connection object before it is automatically freed by the Python GC.
As this is an advanced method, it is left undocumented for now, and is
prefixed with an underscore.

Change-Id: Ic784dcea05bd8697eb934583ff111b3dc413aff8
Reviewed-on: http://review.couchbase.org/28228
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>