PYCBC-61: Get-And-Lock (and unlock)

Change-Id: I994f3b0c8c277bfb368bf688faae040fc8e94c79
Reviewed-on: http://review.couchbase.org/26277
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>