Don't transition stream state when the backfill task completes

Change-Id: Ic97bb65abe0014e8dfaa32828e5e6941fb9751a2
Reviewed-on: http://review.couchbase.org/34112
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>