Properly check the perisistence seqno

If we are in the backfill phase then we need to make sure that we
get the persistence seqno based on where we left off in the backfill
queue. If it is grabbed from the checkpoint queues then it will be
incorrect.

Change-Id: Ife46f8725a7e8f2c2e7298feee88abc7a663f649
Reviewed-on: http://review.couchbase.org/37351
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>