core: re-insert only migrated items

Previously, we were re-inserting all items regardless of whether they
were changed or not. This raised a lot of issues, not to mention
unnecessary load on our servers because after each migration
all the data is re-synced. Since migration takes place independently on
each device, this multiplies exponentially.
Hopefully after this update, this will only happen much more granually.

packages/core/database/backup.js

@@ -201,7 +201,8 @@ export default class Backup {
         this._db,
         collections,
         (id) => data[id],
-        version
+        version,
+        true
       );
     });
   }