Drush should set MAINTENANCE_MODE constant when running updates

See https://www.drupal.org/project/drupal/issues/2910353

That is a critical bug that results in data loss by for example overwriting translations. Drupal will start to throw an exception instead of overwriting the data. To avoid that, it relies on the MAINTENANCE_MODE in the latest patch to never load overrides then.

Drush does not set that constant anymore for D8 and it should do so again.