Adaptive lighting updates (#1679)

This PR simplifies the lighting update system (removes update_type) and instead makes the lighting system decide which update method to use based on server load.

Instant updates are used during low-load, scheduled during high.

Also removes: update_lights_now() (redundant), diff_light() (unused).

code/game/objects/items/devices/flashlight.dm

@@ -24,10 +24,10 @@
 /obj/item/device/flashlight/update_icon()
 	if(on)
 		icon_state = "[initial(icon_state)]-on"
-		set_light(brightness_on, update_type = UPDATE_NOW)
+		set_light(brightness_on)
 	else
 		icon_state = "[initial(icon_state)]"
-		set_light(0, update_type = UPDATE_NOW)
+		set_light(0)
 
 /obj/item/device/flashlight/attack_self(mob/user)
 	if(!isturf(user.loc))
@@ -231,7 +231,7 @@
 
 /obj/item/device/flashlight/slime/New()
 	..()
-	set_light(brightness_on, update_type = UPDATE_NOW)
+	set_light(brightness_on)
 
 /obj/item/device/flashlight/slime/update_icon()
 	return