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/receive_imf.rs

@@ -1034,7 +1034,10 @@ async fn add_parts(
             };
             if update_config {
                 context
-                    .set_config(Config::LastCantDecryptOutgoingMsgs, Some(&now.to_string()))
+                    .set_config_internal(
+                        Config::LastCantDecryptOutgoingMsgs,
+                        Some(&now.to_string()),
+                    )
                     .await?;
             }
         }