MB-9898: Add set vbucket state response api to upr message producers

This is needed as a response to the set vbucket state command since
ep-engine may defer the response to a later time.

Change-Id: Iec534c0b0ca085f729bca55dfca1c58926c9ea6b
Reviewed-on: http://review.couchbase.org/36573
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>