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/

msg/orb_test.msg

@@ -0,0 +1,5 @@
+uint64 timestamp		# time since system start (microseconds)
+
+int32 val
+
+# TOPICS orb_test orb_multitest