MB-11577: Fix to Disk Queue Average Age Stats.

Change-Id: I92f6a14d7962e8ecf926d6bcccaee189f8c86b2d
Reviewed-on: http://review.couchbase.org/38901
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>