async Task cancellation

[maxfischer2781]

Short description of the feature Cancelling a task currently a) allows the task to delay/ignore the cancellation and b) does not wait for completion. This means the cancelling task has to manually account for shutdown (e.g. https://github.com/MatterMiners/lapis/pull/80).

Instead, we should offer an API that cancels a task and waits for completion. Notably, our internal, non-async API (Task.__close__) relies on prompt and successful termination.

Describe the solution you'd like A public, asynchronous method for immediate shutdown. Roughly equivalent to:

class Task:
    ...

    async def terminate(self):
        self.__cancel__()
        await self.done

I'm still not 100% sure if we should use Coroutine.close(via .__close__) or add a cancellation (like .cancel currently does). close has guaranteed semantics, but allows no async action by the Task. A cancellation may be easier to handle in the activity, but requires a check for promptness.