async_client.cpp example can eat all available VM

[egor@…]

As found at ​http://www.rsdn.ru/forum/cpp.applied/3517122.aspx, in russian:

boost::asio::async_read_until(socket_, response_, "\r\n",
          boost::bind(&client::handle_read_status_line, this,
            boost::asio::placeholders::error));

What if the server won't ever return "\r\n" - will response_ grow up to all available RAM?

[chris_kohlhoff]

(In [60686]) Add note to examples on how to limit asio::streambuf growth. Fixes #3370.