Only add new episodes to queue after all feed updates have finished
After making the feed updates async, it occasionally happens that adding the feeds to the queue (which is a write to the DB) errors out because the DB is locked by one of the feed update threads. We can avoid this by only adding new episodes to the queue after all updates have finished. This would also make more sense in other ways: we can add all new episodes chronologically (instead of chronologically per feed in the order that those finish their update).