FlightTasks: add SubscriptionArray class that contains all subscriptions

This is to avoid dynamic (re-)allocations on task switching, due to
orb_{un,}subscribe calls.

src/modules/uORB/Subscription.hpp

@@ -83,6 +83,10 @@ public:
 
 	int getHandle() const { return _handle; }
 
+	const orb_metadata *meta() const { return _meta; }
+
+	unsigned instance() const { return _instance; }
+
 protected:
 	const struct orb_metadata *_meta;
 	int _handle;