requestChunkPopulation() respects the queue size, orderChunkPopulation() does not.
requestChunkPopulation() should be used for non-essential generation (which mainly includes generation for player use).
orderChunkPopulation() should probably be used by plugins.
it's now the World's responsibility to manage adding/removing entities from appropriate chunks. Entities no longer know or care that chunks exist.
Entity->checkChunks() remains as-is for backwards compatibility - now it just calls the world to sync its position.
if a PopulationTask took place after the target worker was garbage collected, the population would fail and the chunks it used would be copied for nothing.
This change marks workers as having unregistered generators when detecting that a worker that previously had a generator registered is restarted.
a non-generated chunk is now always represented by NULL. This forces the case of ungenerated chunks to be handled by all code, which is necessary because ungenerated chunks cannot be interacted with or modified in any meaningful way.
in cases like PopulationTask it makes more sense to store the coordinates separately where they can be stored more efficiently (once instead of 9 times)
In addition, PopulationTask shouldn't need to serialize an empty chunk just to copy coordinates.
I've made changes like this in other areas already in preparation for the day when chunks no longer contain their coordinates, so this brings us one step closer to that goal.
this only made sense to work around the server crashing when creating a player in ungenerated terrain, but spawning at y=256 is hardly preferable, and it'll crash since fd99445c5bbd2a5a6fdd0109d8a196ea6160e0a3 anyway ...
instead, it will fire when the chunk comes out of PopulationTask and is set into the world using setChunk().
There is still one place left where auto-creation of empty chunks is used by the core, and that's an issue i'm still deciding how to deal with.
I'm of two minds whether this should continue to fire onChunkChanged() on chunk loaders or not, since the chunks won't be referenced by any chunk loader anyway by virtue of not existing ...
I think we should probably get rid of this considering the potential for inconsistencies within a chunk, but not retaining this is a bug nonetheless, even though it doesn't have any effect in PM itself since we always use BlockLegacyIds << 4 as the empty block ID.
so, this is only really aiding (ab)use cases which weren't intended anyway ...
this better describes the purpose, which is to identify air.
though, it might make more sense to make air just always have zero as air's runtime ID, since this parameter is apparently making plugin devs think that this is suitable to fill a chunk with a specific block ...
