Implemented durability requirements.

All storage operations now allow for persist_to or replicate_to
options to be passed to them which will cause an internal endure
call to be made to ensure all durability requirements pass before
invoking any user callbacks.

Change-Id: I8f2c2574833d248f013e6bd674fca9b6434006a8
Reviewed-on: http://review.couchbase.org/28839
Tested-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>