release-24.3: changefeedccl: fix unwatched column families memory monitoring bug

[andyyang890]

Backport 1/1 commits from #154802.

/cc @cockroachdb/release

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This patch fixes a bug where the memory monitor wouldn't reclaim the
memory allocated to events corresponding to unwatched column families
for a changefeed that targets only a subset of a table's families.

Fixes #154776

Release note (bug fix): A bug where a changefeed targeting only a subset
of a table's column families could become stuck has been fixed.

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Release justification: fix for high-severity bug

[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.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #155024 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Stability or security issues]
Backward compatible: true
Explanation: The pull request qualifies for a policy exemption due to the inclusion of a valid release justification statement 'Release justification: fix for high-severity bug', which identifies the backport as addressing a high-severity issue. The changes primarily relate to a bug fix in the 'changefeedccl' component, where a memory monitoring issue would lead to the changefeed process becoming stuck. This addresses a critical bug as per the policy since it fixes a bug that can affect the stability of the DB and disrupt its operations. Moreover, the changes are limited to the relevant CCL package and include tests, indicating a targeted and verified fix. There are no feature flags needed since this is a critical bug fix.

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