feat: Pass current runtime handle in ExecutionPhase::Running

[mgs255]

In order to perform any asynchronous operations as a result of receiving the "server is now running" broadcast message I found myself needing access to the current Tokio handle.

In my case I was trying to initialize a memcached client which is used as a 2nd level storage layer, for example:

pub fn init_server(config: &Config) -> Result<Server> {
    // Start pingora server with a much reduced grace period....
    let mut pingora_config = ServerConf::default();
    pingora_config.work_stealing = true;
    pingora_config.grace_period_seconds = Some(15);
    pingora_config.graceful_shutdown_timeout_seconds = Some(10);
    let mut server = Server::new_with_opt_and_conf(None, pingora_config);

    // Set up ExecutionPhase broadcast hook to initialize memcached client as soon as server starts
    let mut phase_watch = server.watch_execution_phase();
    std::thread::spawn(move || {
        while let Ok(phase) = phase_watch.blocking_recv() {
            debug!("Received ExecutionPhase: {:?}", phase);
            if let ExecutionPhase::Running(handle) = phase {
                info!("Server entered Running phase - initializing memcached client");
                // Use the provided runtime handle to spawn the initialization task
                handle.spawn(async {
                    match get_memcached_client().await.version().await {
                        Ok(version) => info!("Memcached client initialized successfully - version: {}", version),
                        Err(e) => error!("Failed to initialize memcached client: {}", e),
                    }
                });
                break;
            }
        }
    });

    server.bootstrap();
    ...

Without this I couldn't find any other nice way of initializing the memcached library nicely before the server starts receiving requests.

Closes: #694

[mgs255]

Hi @andrewhavck

I don't know if you have had a chance to review. Since I first created this I added some other phases where the runtime handle might be usefully passed. In addition I added a bit to the documentation on stop/start as well as an example of usage.