Durable local storage proposal

This is a proposal for #598


We already have a notion of Volume "kinds" in the form of the VolumeSource structure.  Rather than defining a new taxonomy of types, maybe we can start with the notion of lifetimes.

Most Volumes are associated with a particular Pod (they are created together and destroyed together), but it is also possible to give a Volume it's own lifetime (created and destroyed independently of any Pod).