[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.
if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',
ask_confirm=False):
rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',
ask_confirm=False)