Breaking change: new Socket API system

[KalleV]

Update the SocketIOLoader to load socket connection handlers instead
of simply passing the connected socket to an 'onConnect' function. The
connection handler objects allow the developer of a socket API to specify
the following fields:

• event: the event name
• onEvent: the main event handler
• middleware: an optional array of connect-style middleware that run before the onEvent handler is called

Additional:

• Update documentation for the new socket API creation system.
• Update tests