server: use longer-lived context for purgeOldSessions

[stevendanna]

It looks like, the async task start by startPurgeOldSessions outlived the span on the context that it was being passed. Here, we pass it an annotated background context.

Fixes #90692

Epic: None

Release note: None

[cockroach-teamcity]

This change is 

[stevendanna]

Meant to open this as a draft, this is untested.

[knz]

This is likely not sufficient. This PR should import 82e2456 (the 3rd commit from #90608).

[knz]

That said, Andrei and I figured out that the cancel function for the ctx under scrutiny here is not called before the server shuts down, so there is no bug.

edit I was wrong

[stevendanna]

@knz Looks like your PR covers this and more. If you want to fold in the change to tenant.go into your PR (assuming it is correct), we could just merge yours and close this.

[knz]

I'd prefer if you could extract that commit here (and possibly do the change in tenant.go as well), so we can backport your PR and #90523 together as a lighterweight change to 22.2, as requested by @rafiss .

[stevendanna]

@knz I pushed another commit that changes other uses of ctx in startTenantInternal to background. Let me know if any of those don't look correct to you.

[knz]

cheers

[stevendanna]

This hit some CI issues so I couldn't merge it before going offline yesterday. It looks like most of these fixes got merged in other PRs. We'll just need to remember to echo some of those fixes in any backport.

I think the makePlanner thing still needs to be fixed. I'll open a PR for that.