Close connections when using a datasource directly, or don't let it be used directly.

javax.sql.DataSource implements the proto/IConnection protocol, but it's difficult to see how directly using it like that is anything but a misfeature. This protocol implementation lets you do the following:

`(jdbc/fetch datasource "select whatever from whatever;")`

If you do this, the connection that gets created will never get closed. You need to wrap the whole thing in a with-open, jdbc/connection like this:

```
(with-open [conn (jdbc/connection datasource)]
(jdbc/fetch conn "select whatever from whatever;"))
```

Seeing as how the first snippet is always a bad idea, it shouldn't be possible to write the first and have it succeed at all. If we're supposed to go through the connection wrapper jdbc/connection, then the api should force us to do exactly that and not silently succeed when misused.