[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.
# re-read ComputedRelation permissions from schema.py now that we're
# able to serialize them
for computedrtype in schema.iter_computed_relations():
sync_schema_props_perms(computedrtype.type)