Reserved keywords

This PR makes sure that SlmQueue internally uses __name__ and __id__ so that if people set name or id as metadata, it won't conflict with internal usage.

Obviosuly, this break all current jobs... So I'm not sure we should do this (and if we decide to merge this, we should think about other potential changes so that we do not break the payload again in the future).

ping @basz @juriansluiman