first cut at allowing for the ordering of references
not-related ocd
allow multireference property to specify an ordered=True flag, provide a evolve script to change objectid_to_path btree to OO to cope with the fact that objectids may not be integers in the future, change Multireference.oids to Multireference.get_oids so we can override it in tests, have multireference recompute its oids every time it's asked so we don't have to manage the oids list during connect/disconnect