ep_perfsuite: Don't busy-wait if no DCP data available
ep_perfsuite: Don't busy-wait if no DCP data available

In the dcp_client thread function, instead of busy-waiting (i.e. keep
calling dcp->step()) when there's no data available, wait for the
cookie to be notified.

This matches the real-world usage of the dcp step function.

Change-Id: I26710ada88acd3b39159b9a66fbf2b197363b960
Reviewed-on: http://review.couchbase.org/63755
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

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