Use std::atomic<bool> instead of std::atomic_flag unless you have a good reason

[Eisenwave]

I was surprised that we don't have a recommendation on this in the concurrency section.

The gist of it is that you should prefer std::atomic<bool> by default. std::atomic_flag is guaranteed to be lock-free, but it also has a less usual API and is lock-free at all cost, even if that means doing something insanely costly like locking the entire memory bus.