WIP: mysql_innodb_catalog: new subject_set scriptable key
Allows searching the catalog for documents having an exact set of
subjects, not just one subject.
In other words, portal_catalog(subject=["a", "b"]) finds documents
with subject a or subject b and portal_catalog(subject_set=["a, "b"])
finds documents with subject a and subject b.
Edited by Jérome Perrin