Optimize vbucket-details stats call to only return requested keys

Currently, we have vbucket-details call that returns all the keys for respective vbuckets. However, getting all the keys when only few are needed is not optimal. 

The vbucket-details stats call should take the keys requested and only return those key value pairs. 

Due to the fact that ns_server uses vbucket-details to determine state and topology on every janitor cleanup run, this will vastly improve janitor cleanup times and also failover time(calls janitor cleanup).