[Unittest] Fixing unittest test_dcp_consumer_mutate
[Unittest] Fixing unittest test_dcp_consumer_mutate

Wait for consumer buffer items to fall to zero, before
changing the vbucket state to active, to avoid causing
a sporadic failure due to set_vbucket_state racing with
a previously sent snapshot marker.

Change-Id: I06a78abe060111f48cc8a2e41e830c6d6fc9938c
Reviewed-on: http://review.couchbase.org/51516
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

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