Added race test between bg fetch and increment.
Added race test between bg fetch and increment.

The increment actually has to do a bg fetch, so they don't actually
race, but happen in order.

Change-Id: I8cfebcec47c422414b2af0f965c7ff24cd743176
Reviewed-on: http://review.northscale.com/1249
Reviewed-by: Randi Harper <f64102d5a4a6dacf3368157155edc924fc5f86e5@gmail.com>
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

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