Containers that never have children stay forever

ContainerManager does not delete containers whose cversion is zero to avoid situations where a container would be deleted before the application had the chance to create children.

This caused issues in our application where the process stopped between container creation and child creation: the containers were never deleted.

To avoid this while giving applications the time to create children, empty containers with a cversion of zero should be deleted after a grace period, e.g. not when they are first collected, but the second time.