Don't reload classes for every loaded robot.

[kazimuth]

Currently, in order to prevent robots from talking to each other with static variables, we just reload all of the player's classes for every robot. This is slow and memory-intensive. It also means that we have trouble using e.g. Java Collections classes in the RobotController interface, iirc.
If we transparently replaced all loads and stores to static variables in player code with calls to some StaticHolder class, one instance for every robot, maybe with a hash map or a runtime-generated class that holds values, we could eliminate this issue.

cc @jbloxham