this produces a major performance improvement for large render distances, and reduces the impact of lighting calculation to zero on servers which have random blockupdates turned off.
Various bugs existed for a while with stuff using chunk managers instead of worlds when interacting with terrain due to a behavioural inconsistency between World::getChunk() (return from cache or load from disk), and SimpleChunkManager::getChunk() (return from cache only). This change brings the two in line.
World::getOrLoadChunk() has been added as a replacement, which has the same behaviour as the old getChunk() and also makes it more obvious that there is an issue with code using it during refactoring.
this new form allows skipping some useless checks during sky light calculation and also allows getting rid of the last hard dependency on core Block classes.
We're getting real close to native light now.
it's useful to have an immutable stub around for the sake of feeding back dummy read values, but for write values it has to barf instead of being quiet.
There's still some issues with LightArray which I don't currently have a solution for, but I'm thinking about separating light storage from chunks anyway.
copy-on-write and zero-layer SubChunk objects are much easier to manage and have less cognitive overhead.
obviously, the goal is to get rid of EmptySubChunk completely, but right now it does still serve a purpose (filling in dummy values for reading out-of-bounds on chunks), and that's a problem that takes a little more work to fix.
this is more efficient (less data copied for ITC), fixes#2873, and also fixes terrain changes during task run getting overwritten.
This still leaves the problem that the light information provided may be out of date by the time the task completes, but this is nonetheless a step forward.
