Allow deleting completed jobs after a certain number

D7 had:

  $form['advancedqueue_threshold'] = array(
    '#type' => 'select',
    '#title' => t('Number of completed items to keep in the database'),
    '#default_value' => variable_get('advancedqueue_threshold', 0),
    '#options' => array(0 => t('All')) + drupal_map_assoc(array(100, 1000, 10000, 100000, 1000000)),
  );

We can add a similar option to the Database backend, and then perform cleanup in cleanupQueue().