release-22.1: distsql: clean up the determination of txn type in mixed version

[blathers-crl]

Backport 1/1 commits from #78195 on behalf of @yuzefovich.

/cc @cockroachdb/release

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This commit cleans up the way we determine what txn type to use for
a particular flow.

Addresses: #78150.

Release note: None

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Release justification: low risk cleanup.

[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]

Noticed that this wasn't backported when debugging #82783 - I think it's a safe cleanup that makes reasoning about the code simpler, so it's worthy of a backport.

[rharding6373]

I'm not convinced that this is a good candidate to backport. It doesn't fix a bug -- at the time of the original PR, it was also unclear that it would resolve the original issue. And it seems that although it makes debugging easier because the code is easier to reason about, it's not critical for debugging purposes.

On the other hand, it is a simple change and in ~3 months of soak time on the master branch has not been the cause of any issues, so it's low risk.

@rytaft for a second opinion.

[rytaft]

Sorry I missed this earlier. I think @rharding6373 makes a good point here, and I'm generally against backporting stuff without a compelling reason. Do you think this is important to backport, @yuzefovich? Otherwise, I'd suggest we not move forward with this.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @rharding6373 and @yuzefovich)

[yuzefovich]

I have more confidence in the correctness of the code with this patch applied than without, so IMO it's worth it to backport the change from the correctness perspective. That said, this should only matter when the streamer is enabled, which it is not by default on 22.1 branch.

[yuzefovich]

I'm ok with not merging this.

[rytaft]

Oops sorry I never responded to your last comment. I think erring on the side of not backporting non-critical fixes still makes sense. Thanks!