Drupal 8: get and set schema version numbers in database.

check_and_set_schema_version.php

<?php

// All these are found in core/includes/schema.inc
// Get schema version for your module.
drupal_get_installed_schema_version('MY_MODULE');

// schema version number is N in hook_update_N
drupal_set_installed_schema_version('MY_MODULE', 'Schema_version_number');

// Get all schema versions for all modules.
$versions = \Drupal::keyValue('system.schema')->getAll();