Support other databases

[stephenh]

Joist's core model is generally very database-agnostic; the main things that need tackled to support a non-pg database would be:

1. Decouple schema information gathering from pg-structure, which is excellent but pg-only; we'd need like a "db-structure" that supports multiple dbs

2. Rework em.flush to support INSERTs without sequences--currently Joist uses sequences to bulk-assign ids and then do INSERTs/UPDATEs in a dumb/naive order; if we must do the INSERT to know the key, like in MySQL, then that logic needs refactored.

3. Potentially rework em.flush to not rely on deferred foreign keys--currently Joist uses Postgres's deferred foreign keys to, like the previous point, do INSERTs/UPDATEs in a dumb/naive order.