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/sql/migrations.rs

@@ -535,6 +535,16 @@ DO UPDATE SET rfc724_mid=excluded.rfc724_mid,
         )
         .await?;
     }
+    if dbversion < 83 {
+        info!(context, "[migration] v83");
+        sql.execute_migration(
+            "ALTER TABLE imap_sync
+             ADD COLUMN modseq -- Highest modification sequence
+             INTEGER DEFAULT 0",
+            83,
+        )
+        .await?;
+    }
 
     Ok((
         recalc_fingerprints,