Only transition from backfill once all items are sent

The old code allowed an upr stream to transition to in memory once
the backfill task completed even if not all of the backfill items
were sent. This change waits until all of the items are put onto
the wire.

Change-Id: I3f35c5063994743400f87d160125da373b5aaaed
Reviewed-on: http://review.couchbase.org/33162
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>