Rejigger Schema APIs to allow key columns not listed first

In some discussion with mgrund, we came to the conclusion that the current APIs for defining the key columns in a schema are not very easy to understand. Currently, we take only a 'num_key_columns' and require that the columns making up the primary key are the first n columns listed in the column list.

This is a bit confusing to use, and also somewhat restrictive even though conventionally people do list these first. For example, we had to reorder the columns of tpch lineitem to make the primary key columns listed first.