Add indexes to columns used in queries

The attached patch adds the following indexes:

      $schema[$base_table]['indexes'] += [
        // Token value index for direct queries for the unique value in the url.
        'access_token__value' => ['value'],
        // Expiration index for queries to deterine active or expires.
        // @see \Drupal\access_unpublished\AccessTokenManager::buildAccessTokenQuery()
        'access_token__expire' => ['expire'],
        // Entity:Value index for queries looking values for the entity.
        // @see \Drupal\access_unpublished\AccessTokenManager::getAccessTokensByEntity()
        'access_token__entity_value' => ['entity_type', 'entity_id', 'value'],
      ];