Document persistence design

This will eventually become the documentation for persistence design that we'll add to the product website. However, in the meantime, we need to review and discuss it as a team, so here it is.

To view the markdown in a readable format, go to: https://github.com/virtual-world-framework/vwf/blob/branch/persistence-docs/docs/persistence_in_vwf.md

There are still many design questions to be solved in the sections titled "Open Design Questions." I'll also make some better diagrams once we get a little further along.