问什么是ThreadPool服务器的async / await？

TcpListener listener;
void Serve(){
  while(true){
    var client = listener.AcceptTcpClient();
    ThreadPool.QueueUserWorkItem(this.HandleConnection, client);
    //Or alternatively new Thread(HandleConnection).Start(client)
  }
}

async Task Serve(){
  while(true){
    var client = await listener.AcceptTcpClientAsync();
    HandleConnectionAsync(client); //fire and forget?
  }
}