Allocate ChunkSection::changed_blocks on demand

[KonaeAkira]

There is currently a lot of memory being wasted on storing the changed_blocks array even when there are no changed blocks. This PR makes changes to allocate the changed_blocks array on-demand.

Memory consumption when craftgpt is loaded (no players connected):

• Master: ~11.4 GB
• This PR: ~8.6 GB

[StackDoubleFlow]

Did this significantly impact get_block performance at all? I wonder if it will create a redpiler compile time regression, for example.

[KonaeAkira]

I currently don't have the time to test this, but my prediction is that this will have no measurable impact on the performance of get_block.

If someone decides to test this, the test should cover the case where there are no changed blocks and the case where there are changed blocks.