New setting for custom form cache expiration configuration

Drupal's Form API stores details of forms in a cache. By default, these entries are kept for a maximum of 6 hours. We can now change the maximum time in-progress forms are going to be kept in the database, by using a $settings override. Expired entries are cleared by cron.

Example:

To change from 6 hours to only 1 hour, comment out the code in settings.php and do:

$settings['form_cache_expiration'] = 3600;

The value should be a numeric value in seconds.