this restores SimpleChunkManager's behaviour to PM3, removing the need for GeneratorChunkManager (although I'm dubious whether SubChunkExplorer makes any sense in there any more now that we have morton in the mix).
when setBlockSkyLightArray/setBlockLightArray was used, currentLightArray would retain a reference to the old light array, which would cause false readings if SubChunkExplorer didn't move away from that subchunk and back.
This causes a small degradation of performance, but I think it can be implemented differently anyway.
This also fixes#3816.
I accidentally added this during a separation of my local changes, but it's useful anyway, so we should use it.
This removes BlockLightUpdate's implicit dependency on Block, which is a
step towards native light.
