multipart: Code cleanup

- More locking cleanup. Fix naming convention.
- Simplify concatenation and blocking calls.

pkg/fs/definitions.go

@@ -68,7 +68,7 @@ type ObjectMetadata struct {
 	ContentType string
 	Created     time.Time
 	Mode        os.FileMode
-	Md5         string
+	MD5         string
 	Size        int64
 }