message: offboard: make timeout less tight to prevent races
  This is an attempt to make it less likely that we're stopping to send
  setpoints before even having started offboard mode. So basically this
  leaves a user 3 seconds from setting a setpoint to starting offboard.
diff: @@ -621,7 +621,7 @@ void OffboardImpl::process_heartbeat(const mavlink_message_t& message)
  // possibly stale heartbeats for some time.
  std::lock_guard<std::mutex> lock(_mutex);
  if (!offboard_mode_active && _mode != Mode::NotActive &&
  - _time.elapsed_since_s(_last_started) > 1.5) {
  + _time.elapsed_since_s(_last_started) > 3.0) {
  // It seems that we are no longer in offboard mode but still trying to send
  // setpoints. Let's stop for now.
  stop_sending_setpoints();
  