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/naughty-disk_test.go

@@ -28,7 +28,7 @@ import (
 // Programmed errors are stored in errors field.
 type naughtyDisk struct {
 	// The real disk
-	disk *retryStorage
+	disk StorageAPI
 	// Programmed errors: API call number => error to return
 	errors map[int]error
 	// The error to return when no error value is programmed
@@ -39,7 +39,7 @@ type naughtyDisk struct {
 	mu sync.Mutex
 }
 
-func newNaughtyDisk(d *retryStorage, errs map[int]error, defaultErr error) *naughtyDisk {
+func newNaughtyDisk(d StorageAPI, errs map[int]error, defaultErr error) *naughtyDisk {
 	return &naughtyDisk{disk: d, errors: errs, defaultErr: defaultErr}
 }
 
@@ -47,11 +47,11 @@ func (d *naughtyDisk) String() string {
 	return d.disk.String()
 }
 
-func (d *naughtyDisk) Init() (err error) {
-	if err = d.calcError(); err != nil {
-		return err
+func (d *naughtyDisk) IsOnline() bool {
+	if err := d.calcError(); err != nil {
+		return err == errDiskNotFound
 	}
-	return d.disk.Init()
+	return d.disk.IsOnline()
 }
 
 func (d *naughtyDisk) Close() (err error) {