FB 33239 add mutable document revision

See https://gist.github.com/mikerhodes/c022e9e44d83c2e4747e for mini spec (objective-c version)
SQLDatabaseUtils requiring knowledge of the attachment manager class feels a bit upside down. Could just be class naming rather than anything else, but AttachmentManager sits at the MVCC knowledge layer whereas SQLDatabaseUtils clearly lives below that layer.