> Today all transactions are committed on all nodes in the same order, which is a much simpler world.

This is difficult to reconcile with:

> - For the highest performance, you can designate a transaction as "asynchronous" and the master will commit immediately

because if the leader crashes, a replica becomes leader and starts accepting writes, then the old leader recovers as a replica, without something like an epoch number it won't be able to tell that it has commits that the current leader doesn't (using a unique incrementing transaction number based on just a counter at the leader won't work, because it won't necessarily be unique across leader elections thanks to the asynchronous commits).