Set the persistence seqno only when the reject queue is empty

We should only set the seqno a vbucket has persisted up to if
the reject queue is not empty in order to ensure that we don't
miss persisting any items at a lower seqno.

Change-Id: I5d7296897a5c60b5789e2111e3288c2c3137a9ef
Reviewed-on: http://review.couchbase.org/37019
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>