temporary use ep_diskqueue_items

since ep-engine stats are broken

Change-Id: I5b17101e97180dfff2bbc1aee2ad6736aba20a08
Reviewed-on: http://review.couchbase.org/36611
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>