amalthea: use crossbeam channels instead of std channels

[kevinushey]

Rust's std channels have a very annoying property: the Receivers they produce are not Sync. This often implies that if you want to use a Rust std::channel within async code, you need to Arc<Mutex<>> it, even if it's not really shared across different components.

crossbeam's channels do not suffer from this limitation, and are generally regarded as better than the std equivalents. Given that Rust has even decided to import crossbeam-channel to be used instead, I think that's a strong sign that we can use them as well:

rust-lang/rust#93563

I would propose we switch from std channels to crossbeam channels and avoid using the std equivalents.

[kevinushey]

See also: https://docs.rs/tokio/latest/tokio/sync/mpsc/index.html#communicating-between-sync-and-async-code