release-22.1: distsql: create LeafTxn eagerly for local flow of a distributed plan

[blathers-crl]

Backport 1/1 commits from #82829 on behalf of @yuzefovich.

/cc @cockroachdb/release

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This commit makes it so that we eagerly create LeafTxns for local flows
of distributed plans. I believe it was an oversight in d6077d5
(added during 21.2 release cycle) where we started creating LeafTxns
eagerly when the local flow has concurrency. We can do the same thing
for local flows of distributed queries since we know that they must use
LeafTxns anyway.

This oversight was recently exposed by 6c88496
where some processors began storing the txn internally, rather than
accessing it from the flow context. The thing is that we do update the
references to the correct txn from the flow context, but it is done
after the flow setup has been complete, at which point some processors
might have already captured the wrong txn. In particular, this is the
case for a local flow of a distributed query with index / lookup joins
when the streamer API is disabled.

Fixes: #82775.
Fixes: #82776.
Fixes: #82777.
Fixes: #82783.

Release note: None

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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 

[michae2]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rharding6373)