docs: function shutdown(callback) {
    var closed = 0;
    var connected = clients ? clients.length : 0;
  
    function maybeFinished() {
      if(++closed >= connected) {
        clients = null;
        log.info('[Shutdown] All client connections safely closed.');
        return callback();
      }
  
      log.info('[Shutdown] Closed client %s of %s.', closed, connected);
    }
  
    if(!connected) {
      return maybeFinished();
    }
  
    var client;
  
    for(var i = 0; i < connected; i++) {
      client = clients[i] || null;
  
      if(!client) {
        maybeFinished();
      } else {
        client.once('closed', maybeFinished);
  
        if(client.state !== States.CLOSING && client.state !== States.CLOSED) {
          client.close();
        }
      }
    }
  }