uORB: add bitset for faster orb_exists check and remove uORB::Subscription lazy subscribe hack/optimization

- add PX4 bitset and atomic_bitset with testing
 - add uORB::Subscription constructor to take ORB_ID enum
 - move orb test messages into msg/

src/modules/uORB/uORBDeviceNode.hpp

@@ -192,6 +192,8 @@ public:
 
 	const orb_metadata *get_meta() const { return _meta; }
 
+	ORB_ID id() const { return static_cast<ORB_ID>(_meta->o_id); }
+
 	const char *get_name() const { return _meta->o_name; }
 
 	uint8_t get_instance() const { return _instance; }