add_metastring vs get_metastring_id

I prefer the usage of add_metastring over get_metastring_id as the id will be created if it was not already existing. It should not be assumed that get_metastring_id will always return an id. It potentially crashes your code. It was potentially happening in upgrade/2010111501. Other places were not involved, but that was more by accident i think, because the id's were quoted in the queries, and therefore did not crash.