[repo, hooks] small task optimization: don't start cleanup_old_transactions when undo support is not activated, and retrieve a session to update a source only when some source to update is found

sync_schema_props_perms('cw_source', syncprops=False)
add_attribute('CWSource', 'synchronizing')
if schema['BigInt'].eid is None:
    add_entity_type('BigInt')