Before, settling was being processed out of order, so nodes could
fall into the space of nodes below and kick them out as items, so a
solid column of nodes could collapse and eject some nodes as items.
This makes hopper-feeding systems a pain to use.
Instead, explicitly signal upwards (tail-recursive) to settle all
falling nodes on the same tick, since they should already be falling
and accelerating downwards in lock-step.
This isn't perfect, as there are possible race conditions with things
being added into the node-space of the falling column while it's
falling, but it seems to work for now...
