lnwire: make LinkUpdater a subclass of Message

Conceptually, all messages that are capable of identifying the
channel_id on which they operate are themselves messages. Here we
codify this.

lnwire/message.go

@@ -193,6 +193,11 @@ type Message interface {
 // LinkUpdater is an interface implemented by most messages in BOLT 2 that are
 // allowed to update the channel state.
 type LinkUpdater interface {
+	// All LinkUpdater messages are messages and so we embed the interface
+	// so that we can treat it as a message if all we know about it is that
+	// it is a LinkUpdater message.
+	Message
+
 	// TargetChanID returns the channel id of the link for which this
 	// message is intended.
 	TargetChanID() ChannelID