parameters: new parameter server backend

- parameter backend is now running in a wq and accessible over pub/sub
 - parameters library can call into backend directly (with locking), but operations will gradually start moving to pub/sub
 - majority of parameters migrated as is, only small changes to parameter autosave and parameter notification that are now performed from the WQ thread

platforms/common/uORB/SubscriptionBlocking.hpp

@@ -85,6 +85,7 @@ public:
 
 	virtual ~SubscriptionBlocking()
 	{
+		unregisterCallback();
 		pthread_mutex_destroy(&_mutex);
 		pthread_cond_destroy(&_cv);
 	}

platforms/common/uORB/uORBDeviceNode.cpp

@@ -514,7 +514,7 @@ uORB::DeviceNode::register_callback(uORB::SubscriptionCallback *callback_sub)
 		ATOMIC_ENTER;
 
 		// prevent duplicate registrations
-		for (auto existing_callbacks : _callbacks) {
+		for (const auto &existing_callbacks : _callbacks) {
 			if (callback_sub == existing_callbacks) {
 				ATOMIC_LEAVE;
 				return true;