… unawaited coroutine warning

…call_later

it's very easy to accidentally call "await group.call_soon(set_flag)"
which is always wrong, this is especially bad because some type checkers
will warn you that you didn't await your awaitables here

also this prevents issues like
"task = group.call_soon(set_flag); ... await task" where it's easy to
confuse a cancellation coming from asyncio.current_task().cancel() and
task.cancelled() which should be handled differently

delayed already has a meaning in dask so we should
come up with a new name before making it public

this seems to have some difficult to understand consequences
and should be handled in dask#6548

…d state

…the finished event to prevent hangs