Create a store and store/v2 implementation for postgres

[echlebek]

Create implementations of the store, and the store/v2 API, for postgresql.

Alternatively, deprecate the store interface, use store/v2 throughout, and only create a store/v2 implementation.

We should discuss the pros and cons of each approach.

[c-kruse]

Work in progress document capturing some of the options here: https://docs.google.com/document/d/1OfT6fLIqKggGt0pKcFiBGa6Jb5KgygX7DpSy9Pt0XSo/edit?usp=sharing

[echlebek]

This is done as of #4915