message: mission: unregister only on destruction
  We should not unregister stuff just because a vehicle timed out,
  otherwise we leave things unfinished in the case of a connection loss,
  and a user might wait forever for a callback with a result.
diff: @@ -58,13 +58,11 @@ void MissionImpl::init()
  void MissionImpl::enable() {}
  -void MissionImpl::disable()
  -{
  - _parent->unregister_timeout_handler(_timeout_cookie);
  -}
  +void MissionImpl::disable() {}
  void MissionImpl::deinit()
  {
  + _parent->unregister_timeout_handler(_timeout_cookie);
  _parent->unregister_all_mavlink_message_handlers(this);
  }
  