Add stream stats for items read from disk/memory

Change-Id: Ic584fe3bdff2121e0cf8e6f2181d8d0908d6c3ee
Reviewed-on: http://review.couchbase.org/31594
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>