Rework the pathfinder path computation setup for threading.

Essentially reverts D1918 / rP22902.
Instead of copying path requests to workers, setup the result vector, then setup an index, and compute 'in-place'.
To send messages, the result vectors are read in order. This makes the order trivially constant no matter how many workers there are, and the architecture overall makes it much easier to efficiently paralellise.

Tested by: Langbart, Stan

Differential Revision: https://code.wildfiregames.com/D3849