Fix sporadic failure in unit tests when getting upr takeover estimates
Fix sporadic failure in unit tests when getting upr takeover estimates

The estimates might not be valid immediately because the backfill
task needs to bescheduled onto a background thread. As a result we
should wait for the estimate to be correct as opposed to just
checking it once.

Change-Id: I540cf76194e19018be934f43e097143b01df3e12
Reviewed-on: http://review.couchbase.org/37142
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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