ThreadPoolScheduler shutdown and pending tasks.

[Aidan63]

Pending question from the original PR, what should happen to tasks which have been scheduled when the underlying thread pool gets shutdown. My worry is that if the threadpool gets shutdown and there are pending coroutine continuations then the coroutine is essentially deadlocked.

[Simn]

The way this works right now is that the pool simply throws an exception if you try to dispatch something after it has been shut down. It is still going to execute all events that were enqueued before the shutdown occurred, which seems like the correct approach.

I think the only tricky scenario is if we don't control the pool, but run the blocking loop. In that case it could indeed be the case that the blocking thread just sits in the loop after all worker threads terminated. I'm not sure if this is a realistic scenario though because I struggle to see in which setup we would be aware of the scheduler (to run the loop), but unaware of the pool.