Synchronize seen status across devices

Seen status is only synchronized on servers supporting IMAP CONDSTORE
extension. At the end of fetch loop iteration, flags are fetched for
all messages modified since previous synchronization and highest
modification sequence is stored into `imap_sync` table.

src/imap/select_folder.rs

@@ -93,7 +93,11 @@ impl Imap {
         // select new folder
         if let Some(folder) = folder {
             if let Some(ref mut session) = &mut self.session {
-                let res = session.select(folder).await;
+                let res = if self.config.can_condstore {
+                    session.select_condstore(folder).await
+                } else {
+                    session.select(folder).await
+                };
 
                 // <https://tools.ietf.org/html/rfc3501#section-6.3.1>
                 // says that if the server reports select failure we are in