Graceful shutdown: Handle SIGTERM

[thinkharderdev]

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Right now the executors will gracefully shutdown (deregister with the executor) in response to a SIGINT:

    let (notify_scheduler, stop_reason) = tokio::select! {
        service_val = check_services(&mut service_handlers) => {
            let msg = format!("executor services stopped with reason {:?}", service_val);
            info!("{:?}", msg);
            (true, msg)
        },
        _ = signal::ctrl_c() => {
             // sometimes OS can not log ??
            let msg = "executor received ctrl-c event.".to_string();
             info!("{:?}", msg);
            (true, msg)
        },
        _ = stop_recv.recv() => {
            (false, "".to_string())
        },
    };

On kubernetes when a pod terminates it will receive a SIGTERM which doesn't appear to be handled so the executor will not gracefully shutdown.

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Graceful shutdown on unix systems should initiate graceful shutdown on SIGTERM

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[thinkharderdev]

cc @mingmwang @yahoNanJing

[mingmwang]

@thinkharderdev Sorry, I didn't get a chance to test it in a K8s cluster. I will take a look today.

[mingmwang]

#278