[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 repo.system_source.dbdriver == 'postgres':
    sql('ALTER TABLE appears ADD COLUMN weight float')
    sql('UPDATE appears SET weight=1.0 ')