[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.