throw exception on cas mismatch in replace.

Change-Id: I5d92e73d1c99da492df18b665852a25318749997
Reviewed-on: http://review.couchbase.org/40527
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>