[server] use sqls_create_multicol_unique_index to index entities.extid
SQL Server's UNIQUE is not standards compliant, it doesn't accept two
NULL values. Related to #7173702.
for table, column in [
('transactions', 'tx_time'),
('tx_entity_actions', 'tx_uuid'),
('tx_relation_actions', 'tx_uuid')]:
repo.system_source.create_index(session, table, column)
commit()