title: Get event when tcp connection closed in boost::asio
body: <p>I write a tcp server by boost::asio libs and in my application I have object that tend to a client each client have a manager class.
  How can I to know when a tcp connection closed delete the manager object.
  I think to use some thing like <code>async_close(handl closehandler)</code>. that on close socket call to a handler.</p>
  
  <p>if something like this exist in boost?</p>
  
  <p>if this good way to do that or exist other traditional solutions?</p>