Async tls client fix. New server fixture and tests. Don't rely on close_notify.

[themighty1]

This PR:

• introduces fixes in tls-client-async and tests.
• adds a new tls server fixture (with the existing hyper-based one it was not possible to test certain server behaviour).
• does not treat absence of close_notify as an error.

[sinui0]

I'm questioning why we require a timer here? I would find it surprising if the server simply holds the connection open after the client sends a close notify and closes the underlying transport (TCP FIN). In the case that it does hang, the time out functionality can be applied outside this context (in the binary) and probably should be a fatal error.

Building the timeout into the library gives us the platform compatibility problem

[themighty1]

thanks, I agree that it is better to handle the timeout in the outer context, I removed it.

[sinui0]

👍