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/ping.go

@@ -48,9 +48,11 @@ func (p *Ping) Decode(r io.Reader, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (p *Ping) Encode(w *bytes.Buffer, pver uint32) error {
-	return WriteElements(w,
-		p.NumPongBytes,
-		p.PaddingBytes)
+	if err := WriteUint16(w, p.NumPongBytes); err != nil {
+		return err
+	}
+
+	return WritePingPayload(w, p.PaddingBytes)
 }
 
 // MsgType returns the integer uniquely identifying this message type on the