PYCBC-37: Durability requirements

Change-Id: Ic476ef4347c1a8dd112edda2bda57a5f9195f365
Reviewed-on: http://review.couchbase.org/27830
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>