feat(cu): support using PostgreSQL/SQLite interoperability via configuration

[TillaTheHun0]

We should enable persistence interoperability between SQLite and Postgres. SQLite will be the default, but by setting DB_URL to a postgres connection string ie. postgres:// the CU should instead use the Postgres instance for persistence.

I think we might need to use a query builder like knex, in lieu of raw queries, to make interoperability between the two easier in the implementation. But we can determine that when we go to implement.