CCBC-145 Durability operations

This changeset inevitably contains some of the basis for an
'lcb_check' command to be implemented atop observe.

Change-Id: I81d39955a2a987b7327b0bc780d0c45ed1455fc0
Reviewed-on: http://review.couchbase.org/23591
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>