Refactor rusty_backend function to properly return results & move main player loop to own function

[hasezoey]

This PR does:

• convert rusty_backend::queue_next to always return Results instead of just logging error!
• dont construct a new tokio runtime, use the current one already available*1
• move server's main command handling loop to own function
• move server's main command handling loop to be its own thread instead of tokio's spawn_blocking (more adhering to)

*1 in the future, we could convert that function to also be async

[hasezoey]

i still dont know why the macos CI randomly fails in clippy stage