lnwire: add new SerializedSize method to all wire messages

This'll be useful for the bandwidth based rate limiting we'll implement
in the next commit.

lnwire/pong.go

@@ -39,6 +39,9 @@ func NewPong(pongBytes []byte) *Pong {
 // A compile time check to ensure Pong implements the lnwire.Message interface.
 var _ Message = (*Pong)(nil)
 
+// A compile time check to ensure Pong implements the lnwire.SizeableMessage interface.
+var _ SizeableMessage = (*Pong)(nil)
+
 // Decode deserializes a serialized Pong message stored in the passed io.Reader
 // observing the specified protocol version.
 //
@@ -64,3 +67,10 @@ func (p *Pong) Encode(w *bytes.Buffer, pver uint32) error {
 func (p *Pong) MsgType() MessageType {
 	return MsgPong
 }
+
+// SerializedSize returns the serialized size of the message in bytes.
+//
+// This is part of the lnwire.SizeableMessage interface.
+func (p *Pong) SerializedSize() (uint32, error) {
+	return MessageSerializedSize(p)
+}