MB-11801: Seqno persist command should return when persisted seqno == seqno

If the seqno that we are waiting for is already persisted we should return
immediately. This will significantly reduce empty rebalance times.

Change-Id: I7e7437b8a3a370a985149a3d85f6ba250fd2385f
Reviewed-on: http://review.couchbase.org/40063
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>