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/sql/migrations.rs

@@ -368,7 +368,7 @@ UPDATE chats SET protected=1, type=120 WHERE type=130;"#,
         if let Ok(addr) = context.get_primary_self_addr().await {
             if let Ok(domain) = EmailAddress::new(&addr).map(|email| email.domain) {
                 context
-                    .set_config(
+                    .set_config_internal(
                         Config::ConfiguredProvider,
                         get_provider_by_domain(&domain).map(|provider| provider.id),
                     )