Implement monitoring ROS API.

- Add the "query_state" ROS service.

- Add the "state" topic, whose publish rate is configurable.

- Add checks that either error-out or do a no-op when the ROS API is excercised
  in a stopped state. This is important because the ROS API is available when
  the controller is initialized, but not yet running.

- Label realtime and non-realtime methods on public class API.