PYCBC-200, 201, 202: Durability Fixes

This patch adds some minor durability fixes, a context manager for
durability settings, and extended testing for durability embedded in
mutators.

Change-Id: I1989f9e864add7bca0980df9e7938de4eb33c40a
Reviewed-on: http://review.couchbase.org/29641
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>