Avoid calling send() on awaiting native coroutine objects

[m2-farzan]

Fixes #962

Rationale

If a native coroutine (i.e. async def) is to be called manually (i.e. using u = f(); while True: u.send(None), instead of asyncio.create_task), like in the case of our Task class, then the send call should only be done when the coroutine is ready to execute code (i.e. when it has finished an await line). This can be examined by storing the future object returned by the send function.

Disclaimer

Please review carefully (especially if this is to be backported), as I'm not too confident about the side-effects this one might have. Thanks!

[sloretz]

I don't think this can assume a coroutine will return a future. It seems like the behavior is specific to the async library being used (see #962 (comment)). This change is interesting though because it seems like it would both satisfy an rclpy.Future by calling send(None) until it raised StopIteration, and would only call asyncio.Future.__await__ once.

It still needs a way to wake the executor when an asyncio future completes, and given that asyncio isn't thread safe I bet there would be problems if a multithreaded executor was used.

[sloretz]

I'll close this one in favor of #971 for now. It can be reopened if this ends up looking like a better way forward.