release-21.2: pgwire: fix buffer size limiting behavior for addBatch

[DrewKimball]

Backport 1/1 commits from #83870.

/cc @cockroachdb/release

---

Previously, the size of the buffer used to store results before sending them
to the client was checked after buffering each batch entirely. This could
result in significantly exceeding the buffer memory limit when a batch had wide
rows on average.

This patch modifies the pwire logic to ensure that maybeFlush is called each
time a row is buffered. This ensures that the pre-21.2 behavior is preserved.

Fixes #83837

Release note (bug fix): Fixed a bug that was introduced in release 21.2 that
could cause increased memory usage when scanning a table with wide rows.

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.

Some other things to consider:

• 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?

[cockroach-teamcity]

This change is 

[yuzefovich]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball)

[mgartner]

Thanks for backporting! You'll just need to add a release justification to the PR description.

[mgartner]

@DrewKimball @yuzefovich Should we merge this today to get it into 21.2.14?

[yuzefovich]

Yes, it'd be good.

[mgartner]

👍 Thanks!