Durable local storage proposal

This is a proposal for #598


I find these 2 paragraphs to be more obtuse than they have to be.  The distinction between API objects and non is probably not something that someone reading this doc cares about.

Saying "Some *Dir types" is being vague where I don't think you need to.  CachedDirectory is the only one.

The part about multiple copies is unclear what you're trying to express.