Multi-arbiter systems leave lingering Arbiters behind after shutdown.

Expected Behavior

After System::run returns, all system's actors are stopped and dropped, all Arbiters are joined.

Current Behavior

If, inside the system, additional arbiters are created, they outlive the system.

Reproducible example:

#[test]
fn actix_sample() {
    System::run(|| {
        let arb = actix::Arbiter::new();
        A::start_in_arbiter(&arb, |_| A);
        std::thread::sleep_ms(100);
        System::current().stop();
    });
    eprintln!("B");
    std::thread::sleep_ms(300);
}

struct A;

impl Drop for A {
    fn drop(&mut self) {
        std::thread::sleep_ms(200);
        println!("A");
    }
}

impl actix::Actor for A {
    type Context = actix::Context<A>;
}

This prints B, A while it should print A, B. Otherwise, it's not really defined when the actor get's dropped and it might happen after main exits, skipping drops

Context

https://github.com/near/nearcore/issues/3925

Your Environment

• Rust Version (I.e, output of rustc -V): rustc 1.49.0-nightly (91a79fb29 2020-10-07)
• Actix Version: 0.11.0-beta.1