This is needed as explained by @krisis
Lets say we have following errors.
```
[]error{nil, errFileNotFound, errDiskAccessDenied, errDiskAccesDenied}
```
Since the last two errors are filtered, the maximum is nil,
depending on map order.
Let's say we get nil from reduceErr. Clearly at this point
we don't have quorum nodes agreeing about the data and since
GetObject only requires N/2 (Read quorum) and isDiskQuorum
would have returned true. This is problematic and can lead to
undersiable consequences.
Fixes#3298
Disks when are offline for a long period of time, we should
ignore the disk after trying Login upto 5 times.
This is to reduce the network chattiness, this also reduces
the overall time spent on `net.Dial`.
Fixes#3286
- Reads and writes of uploads.json in XL now uses quorum for
newMultipart, completeMultipart and abortMultipart operations.
- Each disk's `uploads.json` file is read and updated independently for
adding or removing an upload id from the file. Quorum is used to
decide if the high-level operation actually succeeded.
- Refactor FS code to simplify the flow, and fix a bug while reading
uploads.json.
