MB-12636: Release the lock before deleting a finished backfill
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 <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>

hisundar/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,membase/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine,couchbase/ep-engine,membase/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,owendCB/ep-engine,owendCB/ep-engine,daverigby/ep-engine,daverigby/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,membase/ep-engine,owendCB/ep-engine,daverigby/kv_engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,membase/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,sriganes/ep-engine,sriganes/ep-engine