Minetest Game's default mod originally allowed other mods to override the
lava-cooling behaviour by simply overriding the default.cool_lava() function.
However, somewhere along the way, a developer that didn't know what they were
doing "optimised" the code, eliminating that functionality. To make the
lava-cooling override work in this mod, I had to have the mod search the ABM
table and modify the lava-cooling function contained in it. It was messy, but
it was just the only way. In Minetest Game 0.4.17, this error has been
corrected. This commit takes advantage of this correction and performs the
override in what was always the correct way to do it, now that that correct
way actually works again.