Durable local storage proposal

This is a proposal for #598


My point in this example was to show that even if a user does not have an automated, containerized data restoration process, they can still use the CachedData concept.

I think what you suggest is certainly possible without any extra concepts: a Pod can have two containers, one which verifies-or-installs the data and then exits, and another which waits for the first to finish and then starts serving.