core: Make sure cgroup_oom_queue is flushed on manager exit

The unit queues are not serialized/deserialized (they are recreated
after reexec/reload instead). The destroyed units are not removed from
the cgroup_oom_queue. That means the queue may contain possibly invalid
pointers to released units.

Fix this by removing the units from cgroup_oom_queue as we do for
others. When at it, sync assert checks with currently existing queues
and put them in order in the manager cleanup code.