this has technically been a longer standing problem, but if an async
chunk loads after a chunk has been removed from the chunk map, it would
be treated as any other spare chunk and kept loaded until Chunk GC kicks in.
This fixes that, but also obsoletes ChunkGC in that anytime we load a spare
chunk (a chunk outside of any players view distance), we will immediately
mark it for unload.
This should reduce the amount of spare chunks loaded on a server.
Vanilla has some screwy logic that doesn't send a chunk until
it has been post processed. This is an issue as post processing
doesn't occur until all neighbor chunks have been loaded.
This can reduce view distance while generating terrain, but also
cause bugs where chunks are never sent to the client.
This fix always sends chunks to the client, and simply updates
the client anytime post processing is triggered with the new chunk data.
