Enrichment concurrency issue with control plane

[NDStrahilevitz]

Since we moved cgroup mkdir and rmdir processing into the control plane, there is an indeterminacy if a cgroup exists by the time its initial enrichment is requested.
The reason is, the control plane signal may arrive later then the cgroup_mkdir event. Since processing is only done from the control plane, enrichment may be enqueued for a mkdir event before the cgroups struct knows of the cgroup.