Add session manager

[BentleyJOakes]

These commits add 'session_manager.js' which handles workers. That is, this session manager code handles:

• Creation of workers
• Handling HTTP calls meant for workers
• Mapping of workers to sockets
• Routing socket messages between the client and between workers.

This frees the HTTP server to only focus on handling generic HTTP messages, and routing worker-specific messages to the session manager.