Add stats for active vs replica GETs

It would be useful to be able to tell what proportion of read operations against a node are for Active rather than Replica vBuckets - for example to see how many replica read operations are occurring.

There are already similar stats for mutations, but not for lookups:

• vb_replica_ops_create
• vb_replica_ops_update
• vb_replica_ops_delete

Could this set be expanded to add a count of the number of lookups - for example vb_active/replica/pending_ops_lookup ?