prune old contract subscriptions

[boudra]

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