Mobs spawn even when they are not supposed to

[DaveyBiggers]

It seems the event we are using to prevent mob spawning isn't the whole story, and/or there are gaps in our state machine which allow the event to go unprocessed.

We should

1. add an event handler for CheckSpawn
2. move that, and the PotentialSpawns event, into the ServerStateMachine, rather than letting them be caught by the individual episodes (which allows some events to fall in the gap between episodes).