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/xl-storage-disk-id-check.go

@@ -64,6 +64,7 @@ const (
 	storageMetricReadXL
 	storageMetricReadAll
 	storageMetricStatInfoFile
+	storageMetricReadMultiple
 
 	// .... add more
 
@@ -510,6 +511,21 @@ func (p *xlStorageDiskIDCheck) StatInfoFile(ctx context.Context, volume, path st
 	return p.storage.StatInfoFile(ctx, volume, path, glob)
 }
 
+// ReadMultiple will read multiple files and send each back as response.
+// Files are read and returned in the given order.
+// The resp channel is closed before the call returns.
+// Only a canceled context will return an error.
+func (p *xlStorageDiskIDCheck) ReadMultiple(ctx context.Context, req ReadMultipleReq, resp chan<- ReadMultipleResp) error {
+	ctx, done, err := p.TrackDiskHealth(ctx, storageMetricReadMultiple, req.Bucket, req.Prefix)
+	if err != nil {
+		close(resp)
+		return err
+	}
+	defer done(&err)
+
+	return p.storage.ReadMultiple(ctx, req, resp)
+}
+
 func storageTrace(s storageMetric, startTime time.Time, duration time.Duration, path string) madmin.TraceInfo {
 	return madmin.TraceInfo{
 		TraceType: madmin.TraceStorage,