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: I82d0abcb1af3657b66b08d31f7f1c31c26534d81
Reviewed-on: http://review.couchbase.org/36298
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>