MB-16307: vb snapshot with no updates should return success
MB-16307: vb snapshot with no updates should return success

At vbucket snapshot time, if there are no updates to the vbucket
state in ForestKVStore, we should return success

Change-Id: I4e4935e212d2689d6b6d181b84dabac71a5da327
Reviewed-on: http://review.couchbase.org/55519
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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