lnwire: add serializing and deserializing for type [6]byte

and remove serialization for type [4]byte.

lnwire/lnwire.go

@@ -171,6 +171,10 @@ func writeElement(w io.Writer, element interface{}) error {
 			return err
 		}
 
+		if _, err := w.Write(e[:]); err != nil {
+			return err
+		}
+	case [6]byte:
 		if _, err := w.Write(e[:]); err != nil {
 			return err
 		}
@@ -467,6 +471,10 @@ func readElement(r io.Reader, element interface{}) error {
 		if _, err := io.ReadFull(r, *e); err != nil {
 			return err
 		}
+	case *[6]byte:
+		if _, err := io.ReadFull(r, e[:]); err != nil {
+			return err
+		}
 	case []byte:
 		if _, err := io.ReadFull(r, e); err != nil {
 			return err