MB-11036: Handling race cond. in session_cas impl

- Description in design document
(link available on ticket)

Change-Id: Ib46717022fc33cf3a1fb324873adc89259f24dc3
Reviewed-on: http://review.couchbase.org/37017
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>