Health-checks

Kafka-streams exposes a method for retrieving the current state: `org.apache.kafka.streams.KafkaStreams#state`.
We use this for implementing health-checks. If `state` is in `created, rebalancing or running` the application is considered healthy. Otherwise the app is considered unhealthy and the application would normally be stopped/restarted.

In PC I've implemented a health-check querying `io.confluent.parallelconsumer.ParallelEoSStreamProcessor#isClosedOrFailed == false`.

Would a different, more "healthcheck-like" abstraction be smart? Perhaps on the `io.confluent.parallelconsumer.ParallelConsumer` interface? The current method requires a cast to ParallelEoSStreamProcessor.

(not critical!)