Support running multiple games / worlds simultaneously

[oubiwann]

Tasks:

• [ ] Refactor configuration for mudstore so that lookups/calls to the backend are done by-game
• [ ] Refactor game server to spawn children dynamically, for each game the server runs
• [ ] Support characters moving between games
• [ ] Support "incarnating" as a different character in another game (e.g., dream state being a different character in a different game)