Prepare for enhancing updating rows

By allowing the loaders to load row data out of postgres on updates.

This is the groundwork for:
  * Update/replace rows option in mutation scripts
      - RowPreparer needs to be given the old row so it can merge the
        update if appropriate.
  * Checking row versions on update
      - New potentially-not-fatal Failure case ("versions didn't match")
      - RowPreparer needs to be given the ability to induce this failure.
  * Using a prepared statement for batched updates
      - Currently each update is a separate Statement.  It'd be better
        to use a prepared statement so they can be batched, but to do
        that, they need to have a consistent shape.