Use `async` where appropriate

[bdarcus]

Premature at this point, but worth considering longer term, including as Rust improves its async support over the next year or two.

https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizing-async-fn-in-trait.html
https://rust-lang.github.io/wg-async/vision/roadmap.html

OTOH, see:

https://blog.hugpoint.tech/avoid_async_rust.html