perf: enable clippy::large_futures lint

Large size of Mimefactor.render() futures
increases the size of all callers
down to set_config() and background_fetch().
I also had to Box::pin one call to fetch_new_msg_batch
and one call to fetch_single_msg.

src/download.rs

@@ -173,9 +173,7 @@ pub(crate) async fn download_msg(
     if msg_transport_id != transport_id {
         return Ok(None);
     }
-    session
-        .fetch_single_msg(context, &server_folder, server_uid, rfc724_mid)
-        .await?;
+    Box::pin(session.fetch_single_msg(context, &server_folder, server_uid, rfc724_mid)).await?;
     Ok(Some(()))
 }