feat: Context::set_config(): Restart IO scheduler if needed (#5111)

Restart the IO scheduler if needed to make the new config value effective (for `MvboxMove,
OnlyFetchMvbox, SentboxWatch` currently). Also add `set_config_internal()` which doesn't affect
running the IO scheduler. The reason is that `Scheduler::start()` itself calls `set_config()`,
although not for the mentioned keys, but still, and also Rust complains about recursive async calls.

src/authres.rs

@@ -216,7 +216,7 @@ async fn update_authservid_candidates(
     if old_ids != new_ids {
         let new_config = new_ids.into_iter().collect::<Vec<_>>().join(" ");
         context
-            .set_config(Config::AuthservIdCandidates, Some(&new_config))
+            .set_config_internal(Config::AuthservIdCandidates, Some(&new_config))
             .await?;
         // Updating the authservid candidates may mean that we now consider
         // emails as "failed" which "passed" previously, so we need to