This implements threading of the pathfinder workers.
Workers are dispatched paths in a round-robin-ish fashion. It's naive, but it works well enough for now.
The Long pathfinder grid is owned by the pathfinder and so safe to use across turns. The obstructions manager' shapes aren't, but the simulation ensures that we don't try to modify them at the wrong time.
This unfortunately introduces a hard order-dependency between obstruction manager and pathfinder.
The largest benefits of this patch are in graphical mode, as shown below. In non-visual replays, you don't gain nearly as much.