(*) direct cross refs redefined as NOT NULL in the database

(*) direct cross refs redefined as NOT NULL in the database
    (e.g. Node:site_id, Person:site_id, Slice:creator_person_id)
    NOTE. I screwed up, this was already in former commit of planetlab4.sql,

(*) cross-ref mechanism reviewed
    could not handle several xrefs with the same type in the same table

(*) undid ad-hoc trick for selecting local/foreign nodes (was named scope)
    had no added-value and was only used in TestPeers anyway