Independent Multipart Uploads (#15346)

Do completely independent multipart uploads.

In distributed mode, a lock was held to merge each multipart 
upload as it was added. This lock was highly contested and 
retries are expensive (timewise) in distributed mode.

Instead, each part adds its metadata information uniquely. 
This eliminates the per object lock required for each to merge.
The metadata is read back and merged by "CompleteMultipartUpload" 
without locks when constructing final object.

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/storage-rest-common.go

@@ -18,7 +18,7 @@
 package cmd
 
 const (
-	storageRESTVersion       = "v46" // Added MinIO version to FileInfo
+	storageRESTVersion       = "v47" // Added ReadMultiple
 	storageRESTVersionPrefix = SlashSeparator + storageRESTVersion
 	storageRESTPrefix        = minioReservedBucketPath + "/storage"
 )
@@ -53,6 +53,7 @@ const (
 	storageRESTMethodVerifyFile     = "/verifyfile"
 	storageRESTMethodWalkDir        = "/walkdir"
 	storageRESTMethodStatInfoFile   = "/statfile"
+	storageRESTMethodReadMultiple   = "/readmultiple"
 )
 
 const (