release-23.1: pkg/kv/kvclient: update per-vCPU concurrency limits

[blathers-crl]

Backport 1/1 commits from #131226 on behalf of @sean-.

/cc @cockroachdb/release

---

Increase the current values (arbitrarily selected) by 6x (also an
arbitrary increase). Annecdotal evidence from customers suggests
this may unlock some workloads where vCPU utilization appears to
"plateau" around 70%, and now allows for ~100% vCPU utilization.
Until #131125 and #131126 are implemented, it is not possible to
directly monitor these incremental improvements to the defaults.

Epic: CRDB-42388
Epic: CRDB-42389

Release note (performance improvement): Increase the per-vCPU
concurrency limits for KV operations. Specifically, increase
kv.dist_sender.concurrency_limit to 384/vCPU (up from 64/vCPU)
and kv.streamer.concurrency_limit to 96/vCPU (up from 8/vCPU).

---

Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
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  here).
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  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[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 

[nvanbenschoten]

@sean- we're not going to be able to backport this change, as it does not meet the backport policy: https://cockroachlabs.atlassian.net/wiki/spaces/ENG/pages/1043235321/Database+Backport+Patch+Policy. We typically only land tuning changes like this in major releases, so that users don't see unexpected step changes during patch release upgrades. cc. @michae2

[sean-]

Serious issues are defined as high-priority or high-severity bugs in existing functionality, or as critical gaps in an operators' ability to detect and troubleshoot serious issues in their clusters. Serious issues either have high business priority or have severe impact if they were encountered by customers. This includes correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, and an inability to detect and debug production issues.

I read that and thought this would've met the threshold under "high business priority," "have sever impact," or "significant performance regression."

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @michae2, @mw5h, and @nvanbenschoten)

[michae2]

I agree that we shouldn't backport this. It's a good change, but there's some risk to it, too (which is that it could possibly reduce performance for some other kind of workload, or could possibly cause an OOM if someone is running with an incorrect GOMEMLIMIT).