Add an option to track namespace contents

The norns service should be capable of distinguishing between tracked and untracked namespaces. A tracked namespace would represent a transient storage backend such as a compute node's local storage, which could be destroyed when/if the node is rebooted, whereas an untracked namespace would represent a long-term storage backend such as the cluster's parallel filesystem, where data is expected to remain resident.

In the case of tracked namespaces, norns should keep track of the contents of these namespaces and notify the administrative controller that they are not empty if a shutdown command is sent (see #10 (closed)).