Method reader

example/cpp20/coroutines/chat_server.cpp:117–134

private:
  awaitable<void> reader()
  {
    try
    {
      for (std::string read_msg;;)
      {
        std::size_t n = co_await boost::asio::async_read_until(socket_,
            boost::asio::dynamic_buffer(read_msg, 1024), "\n", use_awaitable);

        room_.deliver(read_msg.substr(0, n));
        read_msg.erase(0, n);
      }
    }
    catch (std::exception&)
    {
      stop();
    }
  }

  awaitable<void> writer()
  {