[config] cleanup/refactor server sources file values handling * kill former `sources` method, misnamed since we've only the system source configuration in the sources file (and some default admin account information) * introduce new system_source_config and default_admin_config properties to access to the two information in this file Then use one or the other when appropriate

for table, column in [
        ('transactions', 'tx_time'),
        ('tx_entity_actions', 'tx_uuid'),
        ('tx_relation_actions', 'tx_uuid')]:
    session.cnxset.source('system').create_index(session, table, column)

commit()