Set the byseq number iff SET operation succeeds.

Change-Id: I3708c633dd893b0ae94382751159418870177ccb
Reviewed-on: http://review.couchbase.org/37076
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>