fix timers in task

[koonpeng]

Fixes #985.

I am not familiar with the executor logic in rclpy, so this is a hacky fix I did. Instead of passing a new timeout each tick, I added self._spin_until_future_complete_timer and self._spin_until_future_complete_timeout to wake the executor when the timeout is reached. Also added test_timer_in_task.py to test the new behavior.

---

Looking at the code of spin_until_future_complete, it behaves differently depending on if a timeout is provided.

        if timeout_sec is None or timeout_sec < 0:
            while self._context.ok() and not future.done() and not self._is_shutdown:
                self.spin_once_until_future_complete(future, timeout_sec)
        else:
            start = time.monotonic()
            end = start + timeout_sec
            timeout_left = timeout_sec

            while self._context.ok() and not future.done() and not self._is_shutdown:
                self.spin_once_until_future_complete(future, timeout_left)
                now = time.monotonic()

                if now >= end:
                    return

                timeout_left = end - now

This all looks good but furthur investigation of wait_for_ready_callbacks (which is eventually called later down the chain),

        while True:
            if self._cb_iter is None or self._last_args != args or self._last_kwargs != kwargs:
                # Create a new generator
                self._last_args = args
                self._last_kwargs = kwargs
                self._cb_iter = self._wait_for_ready_callbacks(*args, **kwargs)

            try:
                return next(self._cb_iter)
            except StopIteration:
                # Generator ran out of work
                self._cb_iter = None

It creates a new generator whenever _last_args or _last_kwargs is different, the "timeout" path of spin_until_future_complete does exactly that, each spin_once_until_future_complete is passed a different timeout.

My guess is that somehow creating new generators every "tick" causes new "work" to be created, and executing a "work" causes a new generator to be created, resulting in a infinite list of pending "work", which causes the actual task to never be executed.