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>
Set the number of backfill items only when we actually begin backfill

If we get the number of items in then constructor then it is
possible that their might be more items when we actually begin
backfill. This causes a problem when the num items was 0 in the
constructor and then non-zero when we act.

Change-Id: I81b039e0a78ecfe289904bbe13e60c927eaa39cc
Reviewed-on: http://review.couchbase.org/37020
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>