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/event-notifier_test.go

@@ -37,7 +37,7 @@ func TestInitEventNotifierFaultyDisks(t *testing.T) {
 	// remove the root directory after the test ends.
 	defer os.RemoveAll(rootPath)
 
-	disks, err := getRandomDisks(4)
+	disks, err := getRandomDisks(16)
 	if err != nil {
 		t.Fatal("Unable to create directories for FS backend. ", err)
 	}
@@ -64,12 +64,13 @@ func TestInitEventNotifierFaultyDisks(t *testing.T) {
 	notificationXML += "</NotificationConfiguration>"
 	size := int64(len([]byte(notificationXML)))
 	reader := bytes.NewReader([]byte(notificationXML))
-	if _, err := xl.PutObject(minioMetaBucket, bucketConfigPrefix+"/"+bucketName+"/"+bucketNotificationConfig, mustGetHashReader(t, reader, size, "", ""), nil); err != nil {
+	bucketConfigPath := bucketConfigPrefix + "/" + bucketName + "/" + bucketNotificationConfig
+	if _, err := xl.PutObject(minioMetaBucket, bucketConfigPath, mustGetHashReader(t, reader, size, "", ""), nil); err != nil {
 		t.Fatal("Unexpected error:", err)
 	}
 
 	for i, d := range xl.storageDisks {
-		xl.storageDisks[i] = newNaughtyDisk(d.(*retryStorage), nil, errFaultyDisk)
+		xl.storageDisks[i] = newNaughtyDisk(d, nil, errFaultyDisk)
 	}
 	// Test initEventNotifier() with faulty disks
 	for i := 1; i <= 3; i++ {