Add large bucket support for erasure coded backend (#5160)

This PR implements an object layer which
combines input erasure sets of XL layers
into a unified namespace.

This object layer extends the existing
erasure coded implementation, it is assumed
in this design that providing > 16 disks is
a static configuration as well i.e if you started
the setup with 32 disks with 4 sets 8 disks per
pack then you would need to provide 4 sets always.

Some design details and restrictions:

- Objects are distributed using consistent ordering
  to a unique erasure coded layer.
- Each pack has its own dsync so locks are synchronized
  properly at pack (erasure layer).
- Each pack still has a maximum of 16 disks
  requirement, you can start with multiple
  such sets statically.
- Static sets set of disks and cannot be
  changed, there is no elastic expansion allowed.
- Static sets set of disks and cannot be
  changed, there is no elastic removal allowed.
- ListObjects() across sets can be noticeably
  slower since List happens on all servers,
  and is merged at this sets layer.

Fixes #5465
Fixes #5464
Fixes #5461
Fixes #5460
Fixes #5459
Fixes #5458
Fixes #5460
Fixes #5488
Fixes #5489
Fixes #5497
Fixes #5496

cmd/xl-v1-list-objects.go

@@ -110,10 +110,11 @@ func (xl xlObjects) listObjects(bucket, prefix, marker, delimiter string, maxKey
 			var err error
 			objInfo, err = xl.getObjectInfo(bucket, entry)
 			if err != nil {
-				// Ignore errFileNotFound as the object might have got deleted in the interim period
-				// of listing and getObjectInfo()
-				// Ignore quorum error as it might be an entry from an outdated disk.
-				if errors.Cause(err) == errFileNotFound || errors.Cause(err) == errXLReadQuorum {
+				// Ignore errFileNotFound as the object might have got
+				// deleted in the interim period of listing and getObjectInfo(),
+				// ignore quorum error as it might be an entry from an outdated disk.
+				switch errors.Cause(err) {
+				case errFileNotFound, errXLReadQuorum:
 					continue
 				}
 				return loi, toObjectErr(err, bucket, prefix)