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.

deltachat-repl/src/main.rs

@@ -401,7 +401,7 @@ enum ExitResult {
 
 async fn handle_cmd(
     line: &str,
-    mut ctx: Context,
+    ctx: Context,
     selected_chat: &mut ChatId,
 ) -> Result<ExitResult, Error> {
     let mut args = line.splitn(2, ' ');