settings: audit and prevent races on change

@pav-kv [discovered](https://cockroachlabs.slack.com/archives/G01G8LK77DK/p1723161001811399) possibility for races between registering an `SetOnChange ` callback and initially storing the setting value in memory (https://github.com/cockroachdb/cockroach/pull/128217):

e.g.

```
thing := initThing(important.Setting.Get(&settings.SV))
// do stuff
important.Setting.SetOnChange(&settings.SV, thing.updateSetting)
```

This issue is to audit and prevent races of this type.

Jira issue: CRDB-41147