Reverse-sort delete operations, fix for same-name siblings

This fixes the same name siblings (SNS) delete problem, as tested in [phpcr-api-tests PR 96](https://github.com/phpcr/phpcr-api-tests/pull/96).

It's not a definitive fix for all ordering problems, but it makes it possible to delete a batch of SNS in the correct order for Jackalope.