NCBC-715: Hdd.UsedByData overflows when input is greater than int.maxsize

Change-Id: I6625309f690a06af9cb9c3caa0d69b64f92f7107
Reviewed-on: http://review.couchbase.org/42814
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>