release-22.2: changefeedccl: Fix initial scan checkpointing

[blathers-crl]

Backport 1/1 commits from #96995 on behalf of @miretskiy.

/cc @cockroachdb/release

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An over than 2 year old change
(#71848) that added support for checkpointing during backfill after schema change, inadvertently broke initial scan checkpointing functionality

Exacerbating the problem, the existing test
TestChangefeedBackfillCheckpoint continued to work fine. The reason why it was passing was because the test was looking for a checkpoint whose timestamp matched backfill timestamp. The bug involved incorrect initialize/use of 0 timestamp. It just so happens, that after initial scan completes, the rangefeed starts, and the very first thing it does is to generate a 0 timestamp checkpoint. So, the test was observing this event, and continued to work.
This PR does not have a dedicated test because the existing tests work fine -- provided we ignore 0 timestamp checkpoint, which is what this PR does in addition to addressing the root cause of the bug.

Informs #96959

Release note (enterprise change): Fix a bug in changefeeds, where long running initial scans will fail to generate checkpoint. Failure to generate checkpoint is particularly bad if the changefeed restarts for whatever reason. Without checkpoints, the changefeed will restart from the beginning, and in the worst case, when exporting substantially sized tables, changefeed initial scan may have hard time completing.

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Release justification: bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

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• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[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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[cockroach-teamcity]

This change is