[Refactor] Refactor the shutdown channel

[Phoenix500526]

Currently, our curp server will abort all the background tasks via a Shutdown instance. When a Protocol instance is dropped, it will send a message to notify a background task to abort other tasks. Some relevant codes are shown below:

/// Shutdown broadcast wrapper
#[derive(Debug)]
pub(crate) struct Shutdown {
    /// `true` if the shutdown signal has been received
    shutdown: bool,
    /// The receive half of the channel used to listen for shutdown.
    notify: broadcast::Receiver<()>,
}

impl<C: 'static + Command> Drop for Protocol<C> {
     #[inline]
     fn drop(&mut self) {
         // TODO: async drop is still not supported by Rust(should wait for bg tasks to be stopped?), or we should create an async `stop` function for Protocol
        let _ = self.stop_tx.send(()).ok();
     }
 }

However, we don't need to record this shutdown bool variable, since this shutdown channel will only be notified once. Thus, we can use an event listener to replace the Shutdown struct.