FS/Multipart: Allow for parallel uploads of parts with same part number. (#2587)

CBL client does not close connection when the backup process is stopped, this causes
read() on the stream on the server side to block and hence the lock held on the part
is not released. When the backup process is restarted, we again try to lock on the
part and this will block. Using a unique tmp name and not locking it fixes the problem.

cmd/fs-v1-multipart.go

@@ -412,12 +412,8 @@ func (fs fsObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 		return "", InvalidUploadID{UploadID: uploadID}
 	}
 
-	// Hold write lock on the part so that there is no parallel upload on the part.
-	nsMutex.Lock(minioMetaBucket, pathJoin(mpartMetaPrefix, bucket, object, uploadID, strconv.Itoa(partID)))
-	defer nsMutex.Unlock(minioMetaBucket, pathJoin(mpartMetaPrefix, bucket, object, uploadID, strconv.Itoa(partID)))
-
 	partSuffix := fmt.Sprintf("object%d", partID)
-	tmpPartPath := path.Join(tmpMetaPrefix, uploadID+"-"+partSuffix)
+	tmpPartPath := path.Join(tmpMetaPrefix, uploadID+"."+getUUID()+"."+partSuffix)
 
 	// Initialize md5 writer.
 	md5Writer := md5.New()