This was a little more tricky than might be expected because the
initialization process uses the transactor to bootstrap the database.
Since Clojure doesn't accept mutually recursive modules, this
necessitated a third module, namely "db-factory", which uses both "db"
and "transact". While I was here, I started an "api" module, to paper
over the potentially complicated internal module structure for external
consumers. In time, this "api" module may also grow CLJS-specific JS
transformations.
