Ensures GUID uniqueness across all nodes, even if we use the 'Attach to content
type' feature.

This module is a workaround for this issue:
https://www.drupal.org/project/feeds/issues/1539224

To prevent non-unique nodes from being created, we throw an exception in
global_guid_checker_node_presave(). If you see a better (less of a hack)
way to achieve this, please file a bug report.

See the README file for options.