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/

platforms/common/include/px4_platform_common/atomic.h

@@ -72,7 +72,7 @@ public:
 	static_assert(__atomic_always_lock_free(sizeof(T), 0), "atomic is not lock-free for the given type T");
 #endif
 
-
+	atomic() = default;
 	explicit atomic(T value) : _value(value) {}
 
 	/**
@@ -175,9 +175,9 @@ private:
 #ifdef __PX4_QURT
 	// It seems that __atomic_store  and __atomic_load are not supported on Qurt,
 	// so the best that we can do is to use volatile.
-	volatile T _value;
+	volatile T _value{};
 #else
-	T _value;
+	T _value {};
 #endif
 };