release-23.2: tenantcapabilitieswatcher: make the watcher react faster

[blathers-crl]

Backport 3/3 commits from #114719 on behalf of @stevendanna.

/cc @cockroachdb/release

---

Needed for #111637.
Epic: CRDB-26691

Superceeds #112094

Prior to this patch, the tenant info watcher would only react to
changes to system.tenants upon rangefeed cache flushes, which could
be (in default config) up to 3 seconds after the change is committed.

This commit accelerates the behavior by processing updates as soon as
the rangefeed observes the change.

This new behavior is similar to the way that cluster settings changes
are processed immediately in the settings
watcher (pkg/settingswatcher).

In order to handle deletions that occur during errors that aren't
automatically retried inside the rangefeed library (and are instead
retried by the watcher resulting in a new full scan), we emit any
scan-generated rangefeed events at their scan timestamp, allowing us a
means of clearing any stale data from the cache.

Release note: None

---

Release justification: Substantial improvement that reduces the rate of timeouts and flakes in many tests.

[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.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

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).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[yuzefovich]

Agree about letting it bake on master for a week or so.

Reviewed 1 of 1 files at r1, 1 of 1 files at r2, 17 of 17 files at r3, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @dt and @msbutler)

[github-actions]

Reminder: it has been 3 weeks please merge or close your backport!

[msbutler]

@stevendanna still want to merge this?

[stevendanna]

@msbutler Thanks for the reminder. I'll see if we had any follow-up fixes that should be added to this backport and then rebase it.