multipart: Multipart session map now is based on uploadID.

- Fixes initiating parallel uploads, and configs being quickly
  re-written by another incoming request.
- Parallel uploads work smoothly now and return expected behavior.

pkg/fs/fs.go

@@ -46,6 +46,7 @@ type Buckets struct {
 // MultipartSession holds active session information
 type MultipartSession struct {
 	TotalParts int
+	ObjectName string
 	UploadID   string
 	Initiated  time.Time
 	Parts      []PartMetadata