Run most "daily" vehicle tasks at a fixed frequency at day lengths >= 8

This includes: running costs, track sharing costs, breakdown checks,
servicing checks, order checks

src/vehicle_base.h

@@ -615,6 +615,12 @@ public:
 	 */
 	virtual void OnNewDay() {};
 
+	/**
+	 * Calls the periodic handler of the vehicle
+	 * OnPeriodic is decoupled from OnNewDay at day lengths >= 8
+	 */
+	virtual void OnPeriodic() {};
+
 	/**
 	 * Crash the (whole) vehicle chain.
 	 * @param flooded whether the cause of the crash is flooding or not.