Implement arena resetting

[JackNoordhuis]

Arenas should be defined by two corner positions along with some spawn positions. Currently, we only have two separate spawn positions; to add the arena resetting functionality we need to have the two corner positions so we can cache or unload the chunks the arena is within.

Some work also needs to be done regarding chunk sending for arenas, players only need to see the chunks that contain the arena so sending sounding arenas and their battling players is just a waste of CPU time which degrades performance.

We should also probably look into just loading the needed arena chunks and have them saved in memory but not ticked to save the time it takes reading and loading the chunks from disk.