MB-4786 only show anonymous pages as "used"

It can be argued that it's still imperfect, but at least it can be
argued that it's better than what we had before

Change-Id: I9b698a714b11e4afce29cfb0a8fc2ec9cf026a83
Reviewed-on: http://review.couchbase.org/27227
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>