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/local_position_estimator/BlockLocalPositionEstimator.hpp

@@ -112,7 +112,7 @@ class BlockLocalPositionEstimator : public ModuleBase<BlockLocalPositionEstimato
 public:
 
 	BlockLocalPositionEstimator();
-	~BlockLocalPositionEstimator() override;
+	~BlockLocalPositionEstimator() override = default;
 
 	/** @see ModuleBase */
 	static int task_spawn(int argc, char *argv[]);
@@ -338,8 +338,6 @@ private:
 	uint64_t _time_last_land;
 	uint64_t _time_last_target;
 
-	int _lockstep_component{-1};
-
 	// reference altitudes
 	float _altOrigin;
 	bool _altOriginInitialized;