Add client ID, createSession, joinSession

Client now asks for a client ID from the session manager
Client ID passed on client HTTP messages
Client ID used to re-route messages to proper worker
Created createSession and joinSession commands in session_manager
Session manager now sets up (most) communication directly between clients and workers

Still some work to do to remove worker ID knowledge in client, and ensure that client is not setting up any communication in init.js

AToMPM / atompm