Faster tenure starts to reduce miner commit latency

[kantai]

As part of implementing #5835, the default behavior of miners is to wait a little longer before submitting block commits. This is beneficial to miners because if they didn't wait in those cases, they would end up either RBFing their bitcoin commits (raising their TX fees) or producing a reorg-attempt (which stands a good chance of being forbidden by the signer set). Its also beneficial to the network because with fewer unintended reorg attempts, the network as a whole has higher throughput because there's less contention. However, this situation slightly disadvantages miners (because when sortitions are left empty, their mining power decreases and there may be some loss of mining rewards) and stacker-signers (because when sortitions are left empty, there are no PoX rewards).

As it stands right now, there is a trade-off between getting commits in more quickly vs. making the commits more "accurate". However, speeding up tenure starts (i.e., the first block of a new tenure) is a unqualified win in this trade-off: if tenures begin more quickly, miners will submit their first commits more quickly as well (and they will do so without worrying about unintentionally attempting a reorg).