lnwire: use write buffer in Encode methods

This commit changes the WriteElement and WriteElements methods to take a
write buffer instead of io.Writer. The corresponding Encode methods are
changed to use the write buffer.

lnwire/channel_announcement.go

@@ -87,7 +87,7 @@ func (a *ChannelAnnouncement) Decode(r io.Reader, pver uint32) error {
 // observing the protocol version specified.
 //
 // This is part of the lnwire.Message interface.
-func (a *ChannelAnnouncement) Encode(w io.Writer, pver uint32) error {
+func (a *ChannelAnnouncement) Encode(w *bytes.Buffer, pver uint32) error {
 	return WriteElements(w,
 		a.NodeSig1,
 		a.NodeSig2,