XL: listOnlineDisks should use modTime instead of version. (#2166)

This change is needed to make reading from objects future proof
in-terms of handling online disks. Our current counter is not
based on affirmative knowledge and relies on arithmetic sequence
which can lead to bugs.

Using modTime simplifies the understanding of `xl.json` and future
tooling / debugging of the format.

docs/backend/xl/xl.json

@@ -42,7 +42,6 @@
         ],
     },
     "stat": {
-        "version": 0,
         "modTime": "2016-05-24T00:09:40.122390255Z",
         "size": 14824084
     },