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/bucket-handlers.go

@@ -127,6 +127,13 @@ func (api objectAPIHandlers) GetBucketLocationHandler(w http.ResponseWriter, r *
 		return
 	}
 
+	bucketLock := globalNSMutex.NewNSLock(bucket, "")
+	if err := bucketLock.GetRLock(globalObjectTimeout); err != nil {
+		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
+		return
+	}
+	defer bucketLock.RUnlock()
+
 	if _, err := objectAPI.GetBucketInfo(bucket); err != nil {
 		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
 		return
@@ -397,6 +404,13 @@ func (api objectAPIHandlers) PutBucketHandler(w http.ResponseWriter, r *http.Req
 		return
 	}
 
+	bucketLock := globalNSMutex.NewNSLock(bucket, "")
+	if err := bucketLock.GetLock(globalObjectTimeout); err != nil {
+		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
+		return
+	}
+	defer bucketLock.Unlock()
+
 	// Proceed to creating a bucket.
 	err := objectAPI.MakeBucketWithLocation(bucket, "")
 	if err != nil {