release-25.2: backup: only flush per-node progress every 15s

[dt]

Backport 1/1 commits from #151950.

/cc @cockroachdb/release

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Previously this could queue up a large number of small changes to flush, particularly if flushing was slower than updates, potentially causing BACKUP to hang for hours as it drains this queue (of only marginally useful information, in the case of a otherwise completed job that is just writing out the debug info).

Now all the updates that arrive over a 15s window are rolled up before being saved. If the channel becomes full while saving, additional messages may be dropped.

Release note: none.
Epic: none.
Release justification: fixes severe bug that could stall backups.

[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?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[blathers-crl]

✅ PR #152210 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Significant performance regressions]
Backward compatible: true
Explanation: The PR contains a change to a production file (pkg/backup/backup_job.go) but qualifies for an exemption from the standard backport policy requirements because it includes a 'Release justification: fixes severe bug that could stall backups' section. This justification aligns with the CockroachDB Backport Policy under the 'critical bug criteria,' specifically addressing a significant performance issue potentially leading to system stalling. The PR details and change indicate this fix is critical to system performance during backup operations, and thus its inclusion in a backport is justified.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is