[DCP tests]: Fix a bug in dcp takeover test.
[DCP tests]: Fix a bug in dcp takeover test.

Due to a bug in DCP testsuite code, we were not waiting for takeover
items to go below estimated value.

Change-Id: I22dcca1f92fdbd731d78f91bf877484efdfdc3a6
Reviewed-on: http://review.couchbase.org/60165
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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