geofence: lock geofence items during a write transfers

- avoids race conditions when geofence data is updated in flight. During
  a transfer, the geofence module will not check for violations, which
  is done with the new dm_trylock method.
- there is an update counter stored in dataman, and for each write
  transaction this is increased, so that the geofence module can reload
  the data upon data change (after it's unlocked).
- single dm item updates are atomic already, so resetting the polygons
  to 0 does not need locking.

src/modules/mavlink/mavlink_mission.h

@@ -127,6 +127,9 @@ private:
 	int			_mission_result_sub;
 	orb_advert_t		_offboard_mission_pub;
 
+	static uint16_t _geofence_update_counter;
+	bool		_geofence_locked; ///< if true, we currently hold the dm_lock for the geofence (transaction in progress)
+
 	MavlinkRateLimiter	_slow_rate_limiter;
 
 	bool _verbose;
@@ -236,4 +239,8 @@ private:
 	int format_mavlink_mission_item(const struct mission_item_s *mission_item,
 					mavlink_mission_item_t *mavlink_mission_item);
 
+	/**
+	 * set _state to idle (and do necessary cleanup)
+	 */
+	void switch_to_idle_state();
 };