code: function gen(spark, fn) {
      crypto.randomBytes(8, function generated(err, buff) {
        if (err) return fn(err);
  
        fn(undefined, buff.toString('hex'));
      });
    }
docs: Generator of session ids. It should call the callback with a string that
  should be used as session id. If a generation failed you should set an
  error as first argument in the callback.
  
  This function will only be called if there is no id sent with the request.
  
  @param {Spark} spark The incoming connection.
  @param {Function} fn Completion callback.
  @api public