peer: handling for channel shutdown messages

This commit changes the cooperative channel close workflow to comply
with the latest spec. This adds steps to handle and send shutdown
messages as well as moving responsibility for sending the channel close
message from the initiator to the responder.

lnwire/shutdown.go

@@ -13,7 +13,7 @@ type Shutdown struct {
 	// ChannelID serves to identify which channel is to be closed.
 	ChannelID ChannelID
 
-	// Address is the script to which
+	// Address is the script to which the channel funds will be paid.
 	Address DeliveryAddress
 }