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/imap/scan_folders.rs

@@ -89,7 +89,7 @@ impl Imap {
             Config::ConfiguredTrashFolder,
         ] {
             context
-                .set_config(conf, folder_configs.get(&conf).map(|s| s.as_str()))
+                .set_config_internal(conf, folder_configs.get(&conf).map(|s| s.as_str()))
                 .await?;
         }