release-25.4: changefeedccl: apply all checkpoints before flushing

[blathers-crl]

Backport 1/1 commits from #162546 on behalf of @jeffswenson.

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Previously, the checkpoint logic lived in side of the per-span loop. This could result in updating a checkpoint O(spans) times when there is a lagging span in the frontier. This has always been slightly sub-optimal, because we may advance the high water mark multiple times while processing spans, but its particularly costly as of 25.4 because we save individual frontier changes.

Release note (performance improvement): improved the performance on changefeed checkpoint when changefeeds are lagging. Informs: #158913

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Release justification: fixes a checkpoint performance regression introduced in 25.4.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[blathers-crl]

✅ PR #163282 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Significant performance regressions]
Backward compatible: true
Explanation: The pull request addresses a significant performance regression regarding the handling of changefeed checkpointing, which meets the critical bug criteria under 'Significant performance regressions'. The PR body includes a 'Release justification:' line that clearly describes the regression and the performance improvement brought by the change, therefore exempting it from feature flag requirements. The file changed is a production file ('pkg/ccl/changefeedccl/changefeed_processors.go'), and it's related to core changefeed functionality. The changelog entry also labels it as a 'performance improvement.' The compliance is further supported by specifics about improving performance during conditions where the changefeed may lag, aligning convincingly with the PR's claimed performance enhancements.

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[cockroach-teamcity]

This change is