Digging further I noticed that when a client disconnects, the connection between the proxy and the backend seems to say alive, or at least the sockets do not get removed from the agent. From what I understand it is enough to close the socket to have it freed. For this I've tried to add the following listener:

function finish_request (reqhost, reqport, proxy_server, req, res, io_timeout, keep_alive_timeout) {
  ...
  res.addListener('close', function() {
     Logger.info("Client closed the connection");
     preq.abort();
  });
}  

I need to to some more testing but this seems to be working. The above change can be found in this branch.