MB-11414: Producer's state before active stream tasks

- Ensure producer's state is indeed BACKFILLING before
activestream's markDiskSnapshot task.

Change-Id: Ib68bb39100db06d8435997b327ed15439601b03a
Reviewed-on: http://review.couchbase.org/38185
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>