prune old contract subscriptions

at the moment we keep all contracts we subscribe to, which means the getLogs call grows larger and larger without limits

we have a contract pruner `src/contractSubscriptionPruner.ts` that deletes contracts after X days, this doesn't work for all rounds, as some rounds might run longer than that, so it's disabled at the moment

i think we should use the business logic of rounds to our advantage, some ideas:

- we can assign a default reasonable TTL of 6 months to rounds?
- when a round is paid out, we can immediately unsubscribe
- if a QF round doesn't have votes in weeks, we can unsuscribe