//    - has the latest xl.meta but one or more parts are corrupt
//
// 5. __missingParts__       - has the latest copy of xl.meta but has some parts
// missing.  This is identified separately since this may need manual
// inspection to understand the root cause. E.g, this could be due to
// backend filesystem corruption.

// listOnlineDisks - returns
// - a slice of disks where disk having 'older' xl.meta (or nothing)