No way to tell if `AsyncChannel` is closed

[eaigner]

When a channel is closed, send() simply will return instantly.

If you have an API that returns a channel for consumption, the producer has no way of knowing a channel was closed by the consumer. send() should really throw an error instead of just losing all back pressure instantly. That kind of defeats the purpose of a channel.

If any producer and consumer is allowed to call finish(), any producer and consumer must also be able to check if a channel was finished.

This has nothing to do with a 1:1 relationship as stated in #304

Also for instance in go, writing to a closed channel is forbidden and will panic.