Implement SQL storage backend

Some development versions had a column hierarchy.parent instead of the now current hierarchy.parentid. To migrate such databases: ALTER TABLE hierarchy DROP CONSTRAINT hierarchy_parent_hierarchy_fk; DROP INDEX hierarchy_parent_idx; DROP INDEX hierarchy_parent_name_idx; Then for PostgreSQL: ALTER TABLE hierarchy RENAME COLUMN parent TO parentid; Or for Derby (10.4): RENAME COLUMN hierarchy.parent TO parentid; Then for all databases: ALTER TABLE hierarchy ADD CONSTRAINT hierarchy_parentid_hierarchy_fk FOREIGN KEY (parentid) REFERENCES hierarchy ON DELETE CASCADE; CREATE INDEX hierarchy_parentid_idx ON hierarchy (parentid); CREATE INDEX hierarchy_parentid_name_idx ON hierarchy (parentid, name);