Refactor: increase debugability
Refactor: increase debugability

Assert before doing the subtraction so that you can see the values without
having to add back up causing the currentSize to wrap again.

Change-Id: I29524c5e8f3f1cc69555256e72a418c84a70ef72
Reviewed-on: http://review.northscale.com/1121
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

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