if the worker selected previously had a generator registered, but has since been shutdown, the workerStartHook that cleans up generatorRegisteredWorkers won't yet have been called.
This results in the worker being started by the submission of PopulationTask, and the generator doesn't get preemptively registered.
this fixes a bug I encountered when accidentally pressing ctrl+a+d (which inserts a chr(1) character), because it made the server unable to find the command - but still reported an error containing what looked like a valid command (character isn't printable).
if this happened, the index would stay set in activeChunkPopulationTasks, eventually causing the generation queue to jam up completely and non-forced generation to come to a standstill.
When profiling this, I noticed that we spend a stupidly large amount of time creating useless Position objects in the case of update=true, because Vector3->sides() calls Position->getSide(), which calls Position::fromObject(parent::getSide()). This is stupid because the update logic doesn't require Positions anywhere (as evidenced by this change needing no other alterations.
A rough profile shows that this improves setBlock() performance by about 25% in the update=true case, which is a pretty big margin.
As an added bonus, it gets rid of some unrealized cyclic dependencies in World->changedBlocks.
if the request/cancel/re-request happens all in the time before the queue gets drained, chunk hashes may appear multiple times in the queue. We don't want to process them twice if this happens (although it's mostly harmless anyway).
this can happen especially on large render distances when flying fast and changing direction - we decide we don't want the chunk, then, after changing direction and re-ordering chunks, we decide we do want it again, and end up registering a second callback. In this case, we need to ensure that only one of the callbacks gets executed (it doesn't matter which one).
this could happen if a plugin calls setPopulated(true) on a chunk after a request for its population landed in the queue, but before it actually got processed. In that case, the promise would never get fulfilled.
this does not indicate a failure; it indicates that the chunk has already been successfully populated.
In this case, we shouldn't be putting the task back on the queue.
This is skirting around the real bug, which is that requestChunkPopulation() doesn't check if the target chunk is already populated before it creates a new promise that it will be.
