release-24.2: colexec: add session variable to disable eager cancellation

[yuzefovich]

Backport 1/1 commits from #134560.

/cc @cockroachdb/release

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This commit adds a session variable that allows us to disable the eager cancellation that is performed by the parallel unordered synchronizer in local flows in some cases when it transitions into draining state. This will serve as an escape hatch in case we find more issues with this feature.

Informs: #127043.
Informs: #127942.
Epic: None

Release note: None

Release justification: session variable to go around a tricky bug.

[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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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.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[yuzefovich]

Note that the backport wasn't clean, but the changes are even more targeted than on master.

[michae2]

Reviewed 8 of 8 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

[yuzefovich]

blathers backport 24.1 23.2

[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 merge commit from 2982e43 to blathers/backport-release-24.1-134905: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 24.1 failed. See errors above.

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error creating merge commit from 2982e43 to blathers/backport-release-23.2-134905: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.2 failed. See errors above.

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