Add a stats call to just get disk stats

This was requested by the ns_server team in order to reduce the
amount of stats they would have to get sicne before they were
just using 'stats all'. This commit also adds the ability to get
detailed disk stats which shows file sizes per bucket.

Change-Id: I8dd7097a865a7e0142f6d262b85236ba676702cd
Reviewed-on: http://review.couchbase.org/33056
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>