Increase MapBlock::actuallyUpdateDayNightDiff() performance by 2-8x. ok @celeron55

Before patch, function consumes up to ~8% of the main server loop. After, ~0% (below level of 2 places of significance)

src/mapnode.h

@@ -192,6 +192,14 @@ struct MapNode
 	}
 
 	void setLight(enum LightBank bank, u8 a_light, INodeDefManager *nodemgr);
+
+	/**
+	 * Check if the light value for night differs from the light value for day.
+	 *
+	 * @return If the light values are equal, returns true; otherwise false
+	 */
+	bool isLightDayNightEq(INodeDefManager *nodemgr) const;
+
 	u8 getLight(enum LightBank bank, INodeDefManager *nodemgr) const;
 
 	/**