Don't visit hash table for checkpoint stats.
Don't visit hash table for checkpoint stats.

Getting checkpoint stats does not require us to visit hash table
per vbucket, which is a very expensive operation.

Change-Id: If37d7d5920c1e8c85ae6e7c1cef100213921cc26
Reviewed-on: http://review.membase.org/6440
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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