Depends on D4087

Adding logic for appending holder to the backup/log item. We do not need to use anything like append explicitly. It suffices to fetch the target item from the database, add an attachment to it, and then use put to insert it into the database. DynamoDB will automatically override an existing item.

The backup service can be built successfully.

For the full test plan of this functionality that has been broken into multiple diffs, please see D4090.