[th/dns-update-pending-rh2049421] avoid startup-complete and device activation while DNS update pending
DNS updates happen asynchronously. At least depending on the DNS plugin.
For example, both systemd-resolved and dnsmasq plugins need to send D-Bus requests (which are inherently async).
Also, they either need to spawn the process or wait for the service to be D-Bus activated.
While that is still pending, we need to delay "startup complete" and also that devices reach activated state.