Cancel backfill if if a stream is not in the backfill state

Change-Id: Icd62ab631736b602d016500bc0b991a23198fa4d
Reviewed-on: http://review.couchbase.org/31708
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>