message: offboard: replace std::bind with lambda
  Because they say std::bind should be avoided.
diff: @@ -24,10 +24,9 @@ OffboardImpl::~OffboardImpl()
  void OffboardImpl::init()
  {
  - // We need the system state.
  _parent->register_mavlink_message_handler(
  MAVLINK_MSG_ID_HEARTBEAT,
  - std::bind(&OffboardImpl::process_heartbeat, this, std::placeholders::_1),
  + [this](const mavlink_message_t& message) { process_heartbeat(message); },
  this);
  }
  @@ -84,7 +83,9 @@ void OffboardImpl::start_async(Offboard::ResultCallback callback)
  _parent->set_flight_mode_async(
  SystemImpl::FlightMode::Offboard,
  - std::bind(&OffboardImpl::receive_command_result, this, std::placeholders::_1, callback));
  + [callback, this](MavlinkCommandSender::Result result, float) {
  + receive_command_result(result, callback);
  + });
  }
  void OffboardImpl::stop_async(Offboard::ResultCallback callback)
  @@ -97,8 +98,9 @@ void OffboardImpl::stop_async(Offboard::ResultCallback callback)
  }
  _parent->set_flight_mode_async(
  - SystemImpl::FlightMode::Hold,
  - std::bind(&OffboardImpl::receive_command_result, this, std::placeholders::_1, callback));
  + SystemImpl::FlightMode::Hold, [callback, this](MavlinkCommandSender::Result result, float) {
  + receive_command_result(result, callback);
  + });
  }
  bool OffboardImpl::is_active()
  