[repo] make it clear in internal_cnx that security is disabled as explained in its docstring. It was previously relying on internal connection's fake user (InternalManager) for read security and on disabling security hooks for write security. Using the security_enabled context manager is more readable and more reliable: while the older implementation works thanks to the InternalManager associated to the session, custom hooks should rely on session.[read|write]_security being correctly set. This change also allows selecting Password attributes in internal connections without explicitly disabling read security.