Use the new tokio `LocalRuntime` in `actix_rt`

[Sytten]

This is a tracking issue

Tokio is deprecating the LocalSet (tokio-rs/tokio#6741) in favour of a new LocalRuntime (tokio-rs/tokio#6739) which will avoid panics if a user mixes tokio::spawn with tokio::spawn_local/actix_rt::spawn. It should also improve performances.

Once this is released we should migrate to the new runtime.

[Noah-Kennedy]

This is also likely to improve your performance a bit.

[robjtede]

It's also possible that LocalRuntime will be easy enough to use that we can just make actix-rt optional altogether. In particular, if there's a #[tokio::main(flavor = "local")] or similar.

[Noah-Kennedy]

It's also possible that LocalRuntime will be easy enough to use that we can just make actix-rt optional altogether. In particular, if there's a #[tokio::main(flavor = "local")] or similar.

You should leave this in the LocalSet issue thread as well so folks know that the macros are something you care about.

[Sytten]

We will still need the system and arbiter concepts so pretty sure we will keep the crate around for some time.

[robjtede]

We'd only need them for actors. The crate can stay around but it would be removed from the dependency tree of actix-web in a major release.