Allow iteration on a const histo
Allow iteration on a const histo

Change-Id: I6d246f85c27c9c56fa70275af5ac22e1b30254f1
Reviewed-on: http://review.northscale.com/2970
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

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