WT-16258 started failing transactions on standbys when the oldest timestamp moved forward.

Prepared transactions are a little bit special, since if we prepare a transaction successfully, we guarantee that we can complete it.

This raises a few questions:

• Do standbys do prepared transactions at all? If not, is there a step-up scenario where we need to deal with them?
• Should we allow prepared transactions across checkpoint updates?

These are just to get started. There's probably a bunch more.