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-datatypes.go

@@ -276,3 +276,24 @@ func newFileInfo(object string, dataBlocks, parityBlocks int) (fi FileInfo) {
 	}
 	return fi
 }
+
+// ReadMultipleReq contains information of multiple files to read from disk.
+type ReadMultipleReq struct {
+	Bucket       string   // Bucket. Can be empty if multiple buckets.
+	Prefix       string   // Shared prefix of all files. Can be empty. Will be joined to filename without modification.
+	Files        []string // Individual files to read.
+	MaxSize      int64    // Return error if size is exceed.
+	MetadataOnly bool     // Read as XL meta and truncate data.
+	AbortOn404   bool     // Stop reading after first file not found.
+}
+
+// ReadMultipleResp contains a single response from a ReadMultipleReq.
+type ReadMultipleResp struct {
+	Bucket  string    // Bucket as given by request.
+	Prefix  string    // Prefix as given by request.
+	File    string    // File name as given in request.
+	Exists  bool      // Returns whether the file existed on disk.
+	Error   string    // Returns any error when reading.
+	Data    []byte    // Contains all data of file.
+	Modtime time.Time // Modtime of file on disk.
+}