[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.

sync_schema_props_perms('cw_source', syncprops=False)
if schema['BigInt'].eid is None:
    add_entity_type('BigInt')