ConstReadonlyKEY_CREATED: 0A record that did not exist before was created.
ReadonlyKEY_MODIFIED: 1An existing record's value was updated.
ReadonlyKEY_DELETED: 2A record was deleted.
ReadonlyKEY_RANGE_DELETED: 3A range of keys was deleted via OxiaClient.deleteRange.
Kind of change a Notification represents.