release-25.1: distsql: cancel remote flows on quiesce

[yuzefovich]

Backport 1/1 commits from #143990 on behalf of @yuzefovich.

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We recently saw a test timeout where the cluster shutdown was blocked on the outbox goroutine attempting to set up a connection to one of the nodes. We blocked there indefinitely since the remote flows previously didn't respect quiesce signal (we're using the background context there because the remote flows outlive the SetupFlow RPC handler context). This is now fixed.

The bug has been present since forever, and given that we haven't hit this before, it seems like an extreme edge case, so I decided to omit the release note.

Fixes: #143875.

Release note: None

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Release justification: low-risk node shutdown fix.

[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.
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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.
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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 

[michae2]

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