The object cache implementation is XL cache, which defaults
to 8GB worth of read cache. Currently GetObject() transparently
writes to this cache upon first client read and then subsequently
serves reads from the same cache.
Currently expiration is not implemented.
Fixes#1636
New format now generates a UUID and includes it along with
the order of disks. So that UUID is the real order of disks
and on command line user is able to specify disks in any order.
This pre-dominantly solves our dilemma.
```
{
"format" : "xl",
"xl" : {
"version" : "1",
"disk": "00e4cf06-5bf5-4bb5-b885-4b2fff4a7959",
"jbod" : [
"00e4cf06-5bf5-4bb5-b885-4b2fff4a7959",
....
"c47d2608-5067-4ed7-b1e4-fb81bdbb549f",
"a543293e-99f1-4310-b540-1e450878e844",
"18f97cbe-529a-456a-b6d4-0feacf64534d"
]
},
"version" : "1"
}
```