backup: only flush per-node progress every 15s

[dt]

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.

[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.}

[cockroach-teamcity]

This change is 

[jeffswenson]

LGTM

[dt]

bors r+

[craig]

Build succeeded:

• linux_amd64_fips_build
• check_generated_code
• linux_amd64_build
• local_roachtest_fips
• examples_orms
• lint
• local_roachtest
• unit_tests
• docker_image_amd64
• acceptance

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating backport branch refs/heads/blathers/backport-release-24.1-151950: POST https://api.github.com/repos/dt/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch release-24.1 failed. See errors above.

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error creating backport branch refs/heads/blathers/backport-release-24.3-151950: POST https://api.github.com/repos/dt/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch release-24.3 failed. See errors above.

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error creating backport branch refs/heads/blathers/backport-release-25.2-151950: POST https://api.github.com/repos/dt/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch release-25.2 failed. See errors above.

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error creating backport branch refs/heads/blathers/backport-release-25.3-151950: POST https://api.github.com/repos/dt/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch release-25.3 failed. See errors above.

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