Feature cleanup

v3.0.0 removes ID indirection. This is a breaking change.

This includes:

• The first id parameter from request.discover on the server-side. Client IDs are now always the same as socket.id. This feature was generally confusing, broke redis support and led to increased memory usage.
• The redirect features of request.forward on the server-side. This feature did not work correctly due to general signalling information not being forwarded as well as the initial request, so impact is minimal.

All of this functionality is still possible outside of the library using ordinary socket.io messages.