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.
What I don't like is that we're going to create and destroy all these objects on every sync loop.  Shouldn't we be holding the Volume objects somewhere already?  The only time you should need to create a volume cleaner explicitly is when you find a volume and you don't have a spec for it.