Implement update_seq and instance_start_time fields for /{db} endpoint

Also, it's not good to fetch all records and use `array_pop()`. It's much better to use `SELECT MAX(name) ...` . Because `name` will be the same as `last_seq` (see [line 40](https://github.com/dickolsson/drupal-multiversion/blob/8.x-1.x/src/Entity/SequenceIndex.php#L40) in `SequenceIndex` where the first argument to `->add()` is the score.