kv: zone config changes should apply immediately

[andrewbaptist]

Is your feature request related to a problem? Please describe.
After a user changes a ZONE CONFIG using an alter statement, they expect the system to "quickly" process the change. After the work done to address #108795 we eagerly queue up the SpanConfig changes, however due to rate limits and other failures, we don't process 100% of them successfully the first time and we don't retry quickly.

This was exposed from a change to change-replicas/mixed-version to incorrectly no longer pass the COCKROACH_SCAN_MAX_IDLE_TIME=10ms parameter in #127106.

Describe the solution you'd like
Ideally we should be able to remove that parameter and config setting from all the tests (unless they really are testing the scan interval). This mostly works, but we don't have a good roachtest to validate it and as this failure exposed, it doesn't work 100% of the time.

Jira issue: CRDB-40307

Epic CRDB-39952