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/context.rs

@@ -408,7 +408,7 @@ impl Context {
     }
 
     /// Starts the IO scheduler.
-    pub async fn start_io(&mut self) {
+    pub async fn start_io(&self) {
         if !self.is_configured().await.unwrap_or_default() {
             warn!(self, "can not start io on a context that is not configured");
             return;