Add rounds

[juliangruber]

I am surprised we don't need to include the roundIndex when submitting scores. Could the lack of roundIndex make debugging more difficult for us?
spark-rewards has no notion of rounds. What would we need the round index for?

For troubleshooting - e.g. to detect a duplicate submission of scores for the same round, or a round with no scores submitted.

It also allows us to implement idempotency in spark-rewards: we can add pRetry to spark-evaluate and drop duplicate scores for the same round on the spark-rewards side.