Use async mutex instead of std mutex

[lamafab]

Uses the async Mutex, which locks per task rather than per thread.

[ironoa]

Fine for me, would be very interesting if you could add a Rust documentation link

[lamafab]

Fine for me, would be very interesting if you could add a Rust documentation link

Async Rust has some edge cases that aren't entirely clear at first sight. rust-lang/rust#71072 My vscode Rust plugin automatically added the std Mutex import and it slipped my mind that I need to explicitly use the async version one.

You need to be aware that ordinary mutexes insist on being unlocked on the same thread that locked them; and that executors move tasks from one thread to another; and that the solution is not to make ordinary mutexes more complex but to create a new mutex type altogether. These make sense in hindsight, but I'll bet that they leap to mind only for a small group of elite users.

I guess my initial comment about "nothing major" was not quite correct :)