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/systemcmds/tests/CMakeLists.txt

@@ -33,8 +33,10 @@
 
 set(srcs
 	test_adc.c
+	test_atomic_bitset.cpp
 	test_autodeclination.cpp
 	test_bezierQuad.cpp
+	test_bitset.cpp
 	test_bson.cpp
 	test_conv.cpp
 	test_dataman.c