Mark liquid and light updates low priority

src/map.h

@@ -47,6 +47,9 @@ struct MapEditEvent
 	MapNode n = CONTENT_AIR;
 	std::vector<v3s16> modified_blocks; // Represents a set
 	bool is_private_change = false;
+	// Setting low_priority to true allows the server
+	// to send this change to clients with some delay.
+	bool low_priority = false;
 
 	MapEditEvent() = default;