Workaround for unreliable world load / unload events, possibly fixes #4

[ThePixelbrain]

As has been previously mentioned by Player on the IC2 Forums, the IC2 Energy Net breaks because of unreliable world load / unload events. If the events are wrongly ordered or missing, the IC2 Energy Net can break.

WorldData saves a static ConcurrentMap<Integer, WorldData>. Every time something world related is needed (Energy Net, Trading Market, WindSim) it is retrieved by the dimension ID. Entries on the map are removed through world unload events. If a world unload event goes missing or a world load event is executed afterwards, the old, unloaded world will still be referenced. This becomes an issue in the following scenario:

1. World is being unloaded
2. World Unload event fires
3. World Load event fires, but the world is actually unloaded
4. World is being loaded again, new World object gets created
5. WorldData.get() is called and returns the old, dead world object.
6. All Energy Net tiles fail to register to the Energy Net.

I added a workaround to prevent this from happening. I check if the world objects are still identical on world load. If they are not, I instantiate a new WorldData object. I also clear the map completely after a FMLServerStoppedEvent, since no worlds should exist anymore at that time.