The server on which to setup the listener. Additional configuration. Sets up a listener for the `UPGRADE` event on the server, and stores all connected clients in the client list. When clients disconnect, they are removed from this list. The list is a hash object where each key is the _accept key_ sent by the client, and values are the callback functions to send messages to those clients. The name of the event to send. The data, that will be serialised in _JSON_.