Thread the pathfinder computations using the task manager.

The pathfinder computations are run asynchronously (and potentially on the main thread) in-between simulation turns, thus reducing pathfinder-related lag considerably in common cases.

To make this most efficient, the number of paths computed during a turn via MaxSameTurnMoves is reduced from 64 to 20.

This has a hard dependency on the obstruction manager (via the vertex pathfinder) not being modified in-between simulation turn (or to put it more generally on the simulation state not changing outside of turn computation), otherwise results will be non-deterministic and go OOS. This is currently entirely safe (as in, it indeed does not happen that the simulation state changes in-between turn), but future work towards improving simulation sandboxing would be good.

Thanks to Kuba386 for maintaining & improving the patch in 2020
Thanks to everyone who tested the various iterations of this patch.

Fixes #4324

Differential Revision: https://code.wildfiregames.com/D14