Consolidate ABMs

Spread ABM intervals evenly across 1 to 16 seconds
16s ensures no nodes are missed when player walks past
Adjust chance values to compensate, for identical action rates
Combine lavacooling ABMs into one, return to chance = 1
Grass growth: add 'neighbors = "air"' to avoid
processing the thousands of underground dirt nodes
Grass death: Reduce action rate to that of grass growth
Fire: Use chance = 1 for flame extinguishing
and flame removal when mod is disabled

mods/stairs/init.lua

@@ -233,7 +233,7 @@ end
 if replace then
 	minetest.register_abm({
 		nodenames = {"group:slabs_replace"},
-		interval = 8,
+		interval = 16,
 		chance = 1,
 		action = function(pos, node)
 			node.name = minetest.registered_nodes[node.name].replace_name