lnwire+peer: implement new ping/pong messages and behavior

This commit implements the new ping/pong messages along with their new
behavior. The new set of ping/pong messages allow clients to generate
fake cover traffic as the ping messages tells the pong message how many
bytes to included and can also be padded itself.

lnwire/lnwire.go

@@ -174,6 +174,26 @@ func writeElement(w io.Writer, element interface{}) error {
 		if _, err := w.Write(b[:]); err != nil {
 			return err
 		}
+	case PingPayload:
+		var l [2]byte
+		binary.BigEndian.PutUint16(l[:], uint16(len(e)))
+		if _, err := w.Write(l[:]); err != nil {
+			return err
+		}
+
+		if _, err := w.Write(e[:]); err != nil {
+			return err
+		}
+	case PongPayload:
+		var l [2]byte
+		binary.BigEndian.PutUint16(l[:], uint16(len(e)))
+		if _, err := w.Write(l[:]); err != nil {
+			return err
+		}
+
+		if _, err := w.Write(e[:]); err != nil {
+			return err
+		}
 	case ErrorData:
 		var l [2]byte
 		binary.BigEndian.PutUint16(l[:], uint16(len(e)))
@@ -535,6 +555,28 @@ func readElement(r io.Reader, element interface{}) error {
 		if _, err := io.ReadFull(r, *e); err != nil {
 			return err
 		}
+	case *PingPayload:
+		var l [2]byte
+		if _, err := io.ReadFull(r, l[:]); err != nil {
+			return err
+		}
+		pingLen := binary.BigEndian.Uint16(l[:])
+
+		*e = PingPayload(make([]byte, pingLen))
+		if _, err := io.ReadFull(r, *e); err != nil {
+			return err
+		}
+	case *PongPayload:
+		var l [2]byte
+		if _, err := io.ReadFull(r, l[:]); err != nil {
+			return err
+		}
+		pongLen := binary.BigEndian.Uint16(l[:])
+
+		*e = PongPayload(make([]byte, pongLen))
+		if _, err := io.ReadFull(r, *e); err != nil {
+			return err
+		}
 	case *[6]byte:
 		if _, err := io.ReadFull(r, e[:]); err != nil {
 			return err