Implement update_seq and instance_start_time fields for /{db} endpoint

The created time needs to be measured in microseconds, not seconds. You can generate the microtime as an integer the same way we generate sequence ID. See [line 72](https://github.com/dickolsson/drupal-multiversion/blob/8.x-1.x/src/MultiversionManager.php#L72) in `MultiversionManager`.