MB-12160: Use appropriate CAS for comparison with locked items

In the case of setWithMeta/deleteWithMeta commands, the locked item's
CAS value needs to be compared with the incoming mutation's CAS.

Change-Id: Id12a3c4717b18bc41c3d4b7dded99ea215179e9d
Reviewed-on: http://review.couchbase.org/41594
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>