core/cgroup: cache IO accounting data when pruning a cgroup

core/cgroup: cache IO accounting data when pruning a cgroup

When removing a cgroup in unit_prune_cgroup(), read IO metrics to cache
them similar to the existing treatment of the CPU and memory usage data.

Note that we do not do this for the IP metrics as the firewall objects
are only destroyed in unit_free() and thus stay alive long enough to
be read out directly by all interested parties.

Fixes #26988.