[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html