PYCBC-120: Abstract common lcb_wait functionality

This makes lcb_wait operations always be wrapped by the necessary
locking and unlocking of the GIL (if unlock_gil is true)

Change-Id: Icc4b001c9b7cc122b99024039d3d63c36a33d61e
Reviewed-on: http://review.couchbase.org/26549
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>