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.
Would it be reasonable for the EmptyDirectory to be synchronous, since it's teardown is pretty much an rm -r, or should it maintain the preparation with a temporary move to ".deleting~" namespace and call the remove asynchronously? I guess I'm asking if Prepare should be a sub-procedure of TearDown or not..