uORB Subscription callbacks with WorkItem scheduling on publication (#12207)

src/modules/uORB/Subscription.hpp

@@ -48,6 +48,8 @@
 namespace uORB
 {
 
+class SubscriptionCallback;
+
 // Base subscription wrapper class
 class Subscription
 {
@@ -107,6 +109,10 @@ public:
 
 protected:
 
+	friend class SubscriptionCallback;
+
+	DeviceNode	*get_node() { return _node; }
+
 	bool subscribe();
 	void unsubscribe();