imap: do not synchronize Seen flags on unwatched folders

Synchronizing seen flags doubles the time required to scan all
folders. Delta Chat only marks messages as Seen on Inbox or DeltaChat,
so there is no need to check for Seen flag on other folders.

src/scheduler.rs

@@ -188,6 +188,17 @@ async fn fetch_idle(ctx: &Context, connection: &mut Imap, folder: Config) -> Int
                 return InterruptInfo::new(false);
             }
 
+            // Synchronize Seen flags.
+            if let Err(err) = connection
+                .sync_seen_flags(ctx, &watch_folder)
+                .await
+                .context("sync_seen_flags")
+            {
+                connection.trigger_reconnect(ctx).await;
+                warn!(ctx, "{:#}", err);
+                return InterruptInfo::new(false);
+            }
+
             connection.connectivity.set_connected(ctx).await;
 
             // idle