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.
it doesn't serve any practical purpose to keep these separated, particularly since it's getting so difficult to figure out which errors are coming from which levels (since we always use 9, it doesn't really make any difference).
