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-utils.go

@@ -112,10 +112,11 @@ func diskCount(disks []StorageAPI) int {
 // NOTE: collisions are fine, we are not looking for uniqueness
 // in the slices returned.
 func hashOrder(key string, cardinality int) []int {
-	if cardinality < 0 {
-		// Returns an empty int slice for negative cardinality.
+	if cardinality <= 0 {
+		// Returns an empty int slice for cardinality < 0.
 		return nil
 	}
+
 	nums := make([]int, cardinality)
 	keyCrc := crc32.Checksum([]byte(key), crc32.IEEETable)