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/open_channel.go

@@ -164,6 +164,10 @@ type OpenChannel struct {
 // interface.
 var _ Message = (*OpenChannel)(nil)
 
+// A compile time check to ensure OpenChannel implements the lnwire.SizeableMessage
+// interface.
+var _ SizeableMessage = (*OpenChannel)(nil)
+
 // Encode serializes the target OpenChannel into the passed io.Writer
 // implementation. Serialization will observe the rules defined by the passed
 // protocol version.
@@ -335,3 +339,10 @@ func (o *OpenChannel) Decode(r io.Reader, pver uint32) error {
 func (o *OpenChannel) MsgType() MessageType {
 	return MsgOpenChannel
 }
+
+// SerializedSize returns the serialized size of the message in bytes.
+//
+// This is part of the lnwire.SizeableMessage interface.
+func (o *OpenChannel) SerializedSize() (uint32, error) {
+	return MessageSerializedSize(o)
+}