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-common.go

@@ -25,8 +25,8 @@ import (
 // getLoadBalancedDisks - fetches load balanced (sufficiently randomized) disk slice.
 func (xl xlObjects) getLoadBalancedDisks() (disks []StorageAPI) {
 	// Based on the random shuffling return back randomized disks.
-	for _, i := range hashOrder(UTCNow().String(), len(xl.storageDisks)) {
-		disks = append(disks, xl.storageDisks[i-1])
+	for _, i := range hashOrder(UTCNow().String(), len(xl.getDisks())) {
+		disks = append(disks, xl.getDisks()[i-1])
 	}
 	return disks
 }