[DcpProducer] Remove unnecessary queueLock in closeSlowStream
[DcpProducer] Remove unnecessary queueLock in closeSlowStream

Acquire streamsLock instead while looking for the particular
stream.

Change-Id: Icdeb235073a9ba8c30578143b05a09373a4bea81
Reviewed-on: http://review.couchbase.org/57919
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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