Inspect and adjust mutex usage in all crates

When I was first programming Arti, I didn't fully understand the difference between futures::lock::Mutex and std::sync::Mutex. In particular, I believed that I used the futures-aware mutex in a lot of places where it wasn't necessary.

(The practical difference, as I now understand it, is that you need to use futures::lock::Mutex anywhere that you might await when holding a lock. It is okay to use the regular std::sync::Mutex, even in an async function so long as you never await while holding the lock. The contemplated Rust must_not_await feature will make correct usage easier to enforce here.)

It would be a good idea for someone to have a careful look at everyplace in our code that uses either Mutex, and see if it needs to be replaced with the other.