World: Track which chunks are used by each chunk loader.

[dktapps]

Description

Track a set of chunks which each chunk loader is attached to, indexed by chunk loader.

Justification

It's currently necessary for chunk loaders to have their own systems to track which chunks they are using, for example Player->usedChunks.
However, this is fragile and error-prone: it's possible to subscribe a Player as a chunk loader without it knowing.
This leads to a problem with disposal of Player after disconnect, because they cannot unassign themselves from chunks that they don't know they are assigned to.
Another manifestation is when a chunk loader is assigned to chunks in multiple worlds at the same time.

The net result of this change would be effectively equivalent to the current situation in terms of memory impact.
It would improve robustness by centralising loader chunk tracking, and allow for providing additional API features to simplify the use of chunk loaders, in addition to simplifying the implementation of chunk loaders.

[dktapps]

The impact of this issue with regard to players is eliminated by 217f9c5.