Detached tasks spawned on every config generation set and restored the
placeholder DNS without ordering, so a set could back up the placeholder
itself and a later restore reported success while leaving it in place.
Reconcile from the applied config under the lifecycle lock, and restore
whenever the core stops. Keep one backup per network service, never back
up the placeholder, and restore only while the service still holds it.
Scripts verify every read and write and are killed after 5 s.
Refs #8070, #8168
wonfen