Composite primary keys / composite foreign keys

This does not seem to be mentioned in the documentation, and a quick Google search only points to an [unanswered Stack Overflow question from 2014](https://stackoverflow.com/questions/27222706/how-can-i-have-composite-primary-key-schema-in-jugglingdb).

The question is: does JugglingDB support composite keys, both for PKs and for relations?

I'm making that question because we will have to connect to FB dialect 1 databases from a legacy system; in fact, the new system is meant to run alongside it, so, any major database refactorings are out of question.

The issue asks about composite primary and foreign key support across multiple database adapters. Start by examining the schema definition logic in lib/schema.js and the adapter interfaces for each database (e.g., lib/adapters). Look for existing key handling and relation definitions. 'Done' means implementing and testing composite key support for at least one adapter, ensuring it works with relations.

Written by the indexing model from the issue text.