Minecraft notes various stages of fetal development that chunks are in, with the complete ones being "full". Previously, we'd check against a list of states that are not full to skip a chunk, which is obviously not future proof, and broke as Mojang updated their possible range of values for the status field. It's better to just bail if the status is anything other than "full". Additionally, a key called "isLightOn" is in the chunk now. This seemingly determines whether a chunk has been lit. We're also going to check for this, and bail if it's not. In my tests, I have not yet seen a single "full" chunk that does not have "isLightOn" set. Fixes #1558.