When root instance is updated to a new software release URL, it will re-request
all the instances with the new software release URL.
To make sure the new root instance does not appear has ready when it is
re-requested with new software release URL, introduce a promise that will check
that the instances requested by the root instance have the same software
release URL. For now we do this only for Zope instances, because they are
stateless and restart automatically on configuration changes, unlike stateful
instances like mariadb or ZEO that we don't restart automatically (yet ?).