[polygon] gaps in checkpoints and milestones .seg files

[taratorio]

Looks like we have a bug in snapshot retirement process for checkpoints and milestones that results in gaps being created.

This was caught when debugging another issue, the fix for which is blocked on having correct snapshot retirement without gaps - #13689 (more info on that PR's comments). It also explains the non-deterministic changes in checkpoints.seg and milestones.seg file hashes.

It is also caught by a new set of snapshot integrity checks added in #13702

To resolve this we need to:

• find and fix the bug causing these gaps
• regen checkpoints and milestones .seg files by doing rm -rf datadir/snapshots/*borcheckpoints* && rm -rf datadir/snapshots/*bormilestones* && rm -rf datadir/heimdall