uorb: fix several race conditions during topic initialization

Possible race conditions (they all happen between the check of existence
of a topic and trying to create the node):
- single instance, with multiple advertisers during the first advertise:
  both advertisers see the topic as non-existent and try to advertise it.
  One of them will fail, leading to an error message.
  This is the cause for telemetry_status advert failure seen in SITL in
  rare cases.
- multi-instance: subscription to non-existing instance -> px4_open fails,
  and the subscriber tries to create the node. If during that time a
  publisher publishes that instance, the subscriber will get (instance+1)
  (or fails if the max number of instances is exceeded).
  This is a race that goes pretty much unnoticed.
- multi-instance: 2 publishers can get the same instance (if is_published()
  is false in case both have not published data yet).
  This can also go unnoticed.
  Therefore the patch changes where _advertised is set: it is now set
  directly during the advertisement instead of during publication.

src/modules/uORB/uORBDeviceNode.hpp

@@ -164,6 +164,8 @@ public:
 	 * and publish to this node or if another node should be tried. */
 	bool is_advertised() const { return _advertised; }
 
+	void mark_as_advertised() { _advertised = true; }
+
 	/**
 	 * Try to change the size of the queue. This can only be done as long as nobody published yet.
 	 * This is the case, for example when orb_subscribe was called before an orb_advertise.