MB-12636: Release the lock before deleting a finished backfill

The DCPBackfill class contains a reference counter for a Stream
and in the desctructor for the Active stream we take a lock in
the BackfillManager. As a result we must make sure that we do not
hold the BackfillManager lock when deleting a DCPBackfill.

Change-Id: I7aca0efa6fe4208f17095c563c8e23789a7979ef
Reviewed-on: http://review.couchbase.org/43114
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>