[repo] fix extid2entity to ensure connection's has a cnxset else we'll call source.doexec, which attempt to access to cnxset.cu, and crash in some case (eg unittest_cwxmlparser in sobjects/test). Also, manually switch to 'write' mode to ensure cnxset is not released until commit/rollback when some entity is inserted.

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