px4_work_queue: directly support SITL lockstep

- the purpose is to ensure that every WorkItem (and WorkItems scheduled
by WorkItems) is allowed to run to completion every step
 - per workqueue register a lockstep component whenever a work item is
added (if not already registered)
 - once the work queue is empty unregister component

src/modules/ekf2/EKF2.hpp

@@ -224,7 +224,6 @@ private:
 	uORB::SubscriptionCallbackWorkItem _vehicle_imu_sub{this, ORB_ID(vehicle_imu)};
 
 	bool _callback_registered{false};
-	int _lockstep_component{-1};
 
 	bool _distance_sensor_selected{false}; // because we can have several distance sensor instances with different orientations
 	bool _armed{false};