Fix ckb graceful shutdown

[zhangsoledad]

Because the official documentation is misleading, tokio-rs/tokio#4231, resulting in the current graceful shutdown mechanism is actually incorrect, need to fix and explore how to simplify.

[eval-exec]

I'm going to create a tokio::sync::mpsc channel, and let all tokio tasks get the ownership of a Sender, and wait the Receiver in fn main(), when all tokio tasks finished, Sender will be dropped.

And use tokio_util::sync::CancellationToken to notify all tokio tasks to exit.

use tokio::sync::mpsc::{channel, Sender};
use tokio::time::{sleep, Duration};

#[tokio::main]
async fn main() {
    let (send, mut recv) = channel(1);

    for i in 0..10 {
        tokio::spawn(some_operation(i, send.clone()));
    }

    // Wait for the tasks to finish.
    //
    // We drop our sender first because the recv() call otherwise
    // sleeps forever.
    drop(send);

    // When every sender has gone out of scope, the recv call
    // will return with an error. We ignore the error.
    let _ = recv.recv().await;
}

async fn some_operation(i: u64, _sender: Sender<()>) {
    sleep(Duration::from_millis(100 * i)).await;
    println!("Task {} shutting down.", i);

    // sender goes out of scope ...
}

Ref: "Waiting for things to finish shutting down" Section on tokio shutdown