pointSynthesizingIter currently synthesizes points both above and below existing points. We could probably get away with only synthesizing them above existing points, which would also avoid additional reverse peeks in SeekGE` that are likely very slow.

NB: This sort of assumes that synthetic points are tombstones, since we may not want to skip other synthetic points. But we can cross that bridge when we get there.

Jira issue: CRDB-17346

Epic CRDB-2624