Revert "Remove stillAlive code from TUN/TAP conn as it is no longer required with the new deadlines"

This reverts commit eec70bf2f2.

src/tuntap/tun.go

@@ -259,6 +259,7 @@ func (tun *TunAdapter) wrap(conn *yggdrasil.Conn) (c *tunConn, err error) {
 	// Start the connection goroutines
 	go s.reader()
 	go s.writer()
+	go s.checkForTimeouts()
 	// Return
 	return c, err
 }