Stacked on #21232, #21233 and #21235

This simplifies CheckpointExecutor massively, in preparation for
optimizations which will be needed for throughput improvements

TODO

- [x] Antithesis tests - passed an 8 hour test
- [x] PTN tests - no performance or correctness problems observed
- [x] sui-single-node-benchmark tests - performance is comparable with
old version after dialing concurrency down a bit