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/server-main_test.go

@@ -51,13 +51,16 @@ func TestNewObjectLayer(t *testing.T) {
 	}
 	defer removeRoots(disks)
 
+	globalXLSetCount = 1
+	globalXLSetDriveCount = 16
+
 	endpoints = mustGetNewEndpointList(disks...)
 	obj, err = newObjectLayer(endpoints)
 	if err != nil {
 		t.Fatal("Unexpected object layer initialization error", err)
 	}
 
-	_, ok = obj.(*xlObjects)
+	_, ok = obj.(*xlSets)
 	if !ok {
 		t.Fatal("Unexpected object layer detected", reflect.TypeOf(obj))
 	}