lnwire: on Message interface convert Command to MsgType

This commit modifies the Message interface to convert the Command
method to a MsgType method that uses a new set of message type for all
the defined messages. These new messages types nearly exactly match the
message types used within the current draft of the BOLT specifications.

lnwire/update_fail_htlc.go

@@ -120,12 +120,12 @@ func (c *UpdateFailHTLC) Encode(w io.Writer, pver uint32) error {
 	)
 }
 
-// Command returns the integer uniquely identifying this message type on the
+// MsgType returns the integer uniquely identifying this message type on the
 // wire.
 //
 // This is part of the lnwire.Message interface.
-func (c *UpdateFailHTLC) Command() uint32 {
-	return CmdUpdateFailHTLC
+func (c *UpdateFailHTLC) MsgType() MessageType {
+	return MsgUpdateFailHTLC
 }
 
 // MaxPayloadLength returns the maximum allowed payload size for a UpdateFailHTLC