Use minetest.swap_node() instead of mesecons' version, Minor code changes

here and there to facilitate this.

mesecons/services.lua

@@ -10,7 +10,7 @@ mesecon.on_placenode = function (pos, node)
 			mesecon:activate(pos, node)
 		end
 	elseif mesecon:is_conductor_on(node.name) then
-		mesecon:swap_node(pos, mesecon:get_conductor_off(node.name))
+		minetest.swap_node(pos, {name = mesecon:get_conductor_off(node.name)})
 	elseif mesecon:is_effector_on (node.name) then
 		mesecon:deactivate(pos, node)
 	end