Abstract database class, SQL-based version

- [x] Abstract database class, enable injection into etl and query classes, to enable users to write custom Database classes that don't rely on AWS (much of this work is already done)
- [ ] Build out an example SQL-based instance, both as a demo and to explore whether our data model still fits best with NoSQL/DynamoDB

Start by reading the existing abstract Database support in the ETL and query classes, then trace how custom Database classes are injected. Build an example SQL-based Database implementation and use it to evaluate whether the current data model fits SQL as well as NoSQL/DynamoDB; done means the example works as a demonstrable alternative.

Written by the indexing model from the issue text.