Make query builder generalize past `&mut Store`

Right now, the query builder exposes a simple interface that accepts &mut Store. It's pretty restrictive, since Store is a simple type and since taking a mutable reference enforces linear ordering.

This ticket tracks allowing callers to provide something that can yield the pair of Conn and SQLite connection, so that callers that care about their own connection management can take care of things on their own.

As an aside, I'm surprised that this is as restrictive as I am finding it while writing tests!