Replace `Connection` and `Cursor` with `Warehouse` methods

As discussed in this soda-sql issue we prefer to use the Warehouse methods to execute SQL statements since we have control over this API. However, to unblock sodadata/soda-sql#240 we implemented the Connection and Cursor in sodadata/soda-sql#239. After the issue 479 is resolved we can replace the Connection and Cursor with a Warehouse implementation for Spark.