[migration] fix add_relation_type() for symmetric relations
To check if the corresponding SQL table has to created, CWRelationAddOp
tests that the tablename is not in the 'createdtables' session variable
(i.e. the table hasn't been created before in the same transaction) and
that len(rschema.rdefs) is not greater to 1, that is the relation already
existed in the database before this transaction.
In case of symmetric relations, len(rschema).rdefs will be at least 2
since rdefs will store both (subj, obj) and (obj, subj) relation
definitions.