[Metadata] Dublin Core & QDC - caching

[MC-arjen]

In relation to #417

There also should be a caching mechanisme to keep the fields in Drupal and in MM in sync because in MM they can be changed by OAI, FTP etc.

Caching: We want rest call caching in the nodes. Cache every "/asset/{id} [REST]" with drupal cache: cache_get() and cache_set(). Every Node update should update the node rest cache.

cache invalidator: Assets can be updated in MediaMosa (not in Drupal), to make sure the content in Drupal is correct we need a cache validator cron job (hook_cron). Every time it runs, it should do a "/asset [GET]" search call with the last updated date, to get the assets which might have been updated. These assets must be
re-retrieved.

[forgacs]

The synchronization is ready. It was implemented in the hook_cron(). It creates new nodes, if an asset doesn't have node yet. Otherwise it updates the metadata fields of node.