Return `UnknownStream` instead of panic when calling `finish()` on an already closed `SendStream`

[EAimTY]

I think the behavior should be consist of reset(). The panic is caused by

https://github.com/quinn-rs/quinn/blob/4b309cd46b5a68c96c105b00dfbc65216523677f/quinn/src/send_stream.rs#L143C11-L160

which polls and unwraps a used oneshot receiver.

I created a wrapper over a pair of SendStream and RecvStream to use tokio::io::copy_bidirectional on them. Since tokio::io::copy_bidirectional returns error immediately after encountering error on any direction without gracefully shutting down the other direction, I sometimes needs to manually close the other one. However, there is no way to distinguish which one should I close, it is required to explicitly close both one. Unfortunately, the SendStream may already been closed by tokio::io::copy_bidirectional, and calling close on it multiple times would cause panic. Additionally, the panicked thread holds the MutexGuard, which causes the lock poisoning and crashes the whole program.

[djc]

Sounds good to me, want to submit a PR?

[Ralith]

This was fixed in https://github.com/quinn-rs/quinn/pull/1588.