message: core: use references as addresses might change
diff: @@ -20,8 +20,8 @@ public:
  {}
  virtual ~Sender() = default;
  virtual bool send_message(mavlink_message_t& message) = 0;
  - MAVLinkAddress own_address;
  - MAVLinkAddress target_address;
  + MAVLinkAddress& own_address;
  + MAVLinkAddress& target_address;
  };
  class MAVLinkMissionTransfer {
  