Stop a running timeout if it's there, then create a new one.

def _start_timeout():
    """Stop a running timeout if it's there, then create a new one."""
    global _timeout

    LOGGER.debug('Adding a new timeout in %i ms', _timeout_interval)
    _maybe_stop_timeout()
    _timeout = ioloop.IOLoop.current().add_timeout(
        ioloop.IOLoop.current().time() + _timeout_interval / 1000.0,
        _on_timeout)