MB-29926: Fix/Update the definition of some item-count/checkpoint stats
MB-29926: Fix/Update the definition of some item-count/checkpoint stats

Definition amended for:
  vb_active_curr_items
  vb_replica_curr_items
  vb_pending_curr_items
  ep_checkpoint_memory
  ep_checkpoint_memory_unreferenced
  ep_checkpoint_memory_overhead
  ep_active_checkpoint_memory
  ep_active_checkpoint_memory_unreferenced
  ep_active_checkpoint_memory_overhead
  ep_replica_checkpoint_memory
  ep_replica_checkpoint_memory_unreferenced
  ep_replica_checkpoint_memory_overhead
  ep_pending_checkpoint_memory
  ep_pending_checkpoint_memory_unreferenced
  ep_pending_checkpoint_memory_overhead

Change-Id: If55f1998677804aa7223d2569c23cfd6b4c33971
Reviewed-on: http://review.couchbase.org/95191
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine