lnwire: refactor Encode to use specific writers - II

This commit takes another 10 message types and refactors their Encode
method to use specific writers. The following commit will refactor the
rest.

lnwire/shutdown.go

@@ -48,7 +48,15 @@ func (s *Shutdown) Decode(r io.Reader, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (s *Shutdown) Encode(w *bytes.Buffer, pver uint32) error {
-	return WriteElements(w, s.ChannelID, s.Address, s.ExtraData)
+	if err := WriteChannelID(w, s.ChannelID); err != nil {
+		return err
+	}
+
+	if err := WriteDeliveryAddress(w, s.Address); err != nil {
+		return err
+	}
+
+	return WriteBytes(w, s.ExtraData)
 }
 
 // MsgType returns the integer uniquely identifying this message type on the