Let korma work with specs for existing datasources or JNDI references

Korma currently assumes a connection spec will always include values for :subprotocol, :subname, and :classname, but those aren't the only types of specs supported by clojure.java.jdbc.

In particular, JNDI names or explicit DataSource's may also be referenced in a spec. These are typically obtained from within an app server that already efficiently pools them.

The changes in this PR allow us to use the Korma API with those specs as well.