registry: Implement recursive deletes for ldb-backed registry.

registry: Implement recursive deletes for ldb-backed registry.

When deleting a registry key that contains subkeys or values, Windows performs a
recursive deletion that removes any subkeys or values.  This update makes
deletes for an ldb-backed registry consistent with Windows.

Under ldb, the deletion is done using an explicit transaction.  If an error
occurs during the deletion the entire transaction is cancelled, leaving the
registry as it was before the deletions started.
(This used to be commit ca796c8fb10598674a5eef31d15863e79bcf3db8)