Fix memory exception when getting tags

[sofasurfer]

If there are allot of articles the get tags processor fires an memory exception > 500MB.

This can be fixed by getting the ID's directly instead of getting all the article objects.

Would be great if you could add this to the next version.

[sofasurfer]

Ok i replaced 'web' with $parent->get('context_key') this should work for any context.