transactions: Fail early if the threshold cannot be reached anymore

When we have more than three nodes, then it is possible to determine that a transaction cannot succeed anymore even when not all votes were cast yet. If you for example have four nodes with a vote count of one each, and a threshold of three, then it is always possible to decide the outcome after you've recorded three votes. Waiting for the fourth node to arrive is pointless in that case, and instead we can already move ahead with the other nodes.

Implement this idea for our subtransactions.