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-metadata.go

@@ -410,7 +410,7 @@ func (xl xlObjects) readXLMetaParts(bucket, object string) (xlMetaParts []object
 	}
 	// If all errors were ignored, reduce to maximal occurrence
 	// based on the read quorum.
-	readQuorum := len(xl.storageDisks) / 2
+	readQuorum := len(xl.getDisks()) / 2
 	return nil, reduceReadQuorumErrs(ignoredErrs, nil, readQuorum)
 }
 
@@ -438,7 +438,7 @@ func (xl xlObjects) readXLMetaStat(bucket, object string) (xlStat statInfo, xlMe
 	}
 	// If all errors were ignored, reduce to maximal occurrence
 	// based on the read quorum.
-	readQuorum := len(xl.storageDisks) / 2
+	readQuorum := len(xl.getDisks()) / 2
 	return statInfo{}, nil, reduceReadQuorumErrs(ignoredErrs, nil, readQuorum)
 }