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/EKF2Selector.hpp

@@ -173,8 +173,6 @@ private:
 	uint8_t _lat_lon_reset_counter{0};
 	uint8_t _alt_reset_counter{0};
 
-	int _lockstep_component{-1};
-
 	uORB::Subscription _parameter_update_sub{ORB_ID(parameter_update)};
 	uORB::Subscription _sensors_status_imu{ORB_ID(sensors_status_imu)};