Double wall turret placements for stone walls.

Walls should be more helpful as a defensive feature, currently they only serve to limit movement.
Wall garrisoning is the primary defensive benefit of walls, but only allowing eight to garrison in a long wall section means these eight units are quickly dealt with, especially since they are stationary.
Doubling the wall garrison (by adding another row, not by cramming them side-to-side) not only looks cooler, but also allows a more sizable army to occupy walls.
Refs. https://wildfiregames.com/forum/topic/105534-ai-and-the-walls-dilemma/.

Patch by: @real_tabasco_sauce
Differential revision: https://code.wildfiregames.com/D4914
Comments by: @Stan, @wowgetoffyourcellphone