Use drupal_static instead of static to cache options

Problem/Motivation

It is not possible to reset the cache of options because static instead of drupal_static() used. It is can be usefull to reset this cache in cases when a taxonomy was changed during AJAX call and we need to render a field.

Proposed resolution

Use drupal_static https://api.drupal.org/api/drupal/core%21includes%21bootstrap.inc/functi...