Volume reconciliation loop

Implements a reconciliation loop that tears down volumes that are no longer in use by any pod.

The set of valid volumes are determined by iterating through the list of manifests. The set of active, or mounted, volumes are determined by traversing the root working directory path for any existing volumes. These maps are compared, and if an active volume does not have a match, it is unmounted and removed.

This allows a kubelet to properly remove mounted volumes and clean up after itself in the event of a crash/restart.
Why?  If I read this correctly, you're doing this to dodge having a volume object which does not have a corresponding on-disk representation - is that right?  This feels hacky to me, because we still have teardown state which is the inverse of this condition.

We're sort of dancing around the topic of state machine, still.

Or am I mis-reading the intent here?