Migrate SQL source query isn't ordered

> The consensus was that this will not be fixed because it would break highwater system. There is more discussion in the minutes

Couldn't this be done in a way that doesn't break highwater?

I see two simple ways to make this compatible with highwater.

SqlBase does this:

        // Always sort by the high water field, to ensure that the first run
        // (before we have a high water value) also has the results in a
        // consistent order.
        $this->query->orderBy($high_water_field);

So either:

A. Define that a custom ordering may only be applied in the absence of a highwater. Change the code here to throw an exception if there is already a sort order on the query.

B. If there is a sort order already on the query, alter the sort order to add the highwater field first. For example, if the sort is on field 'foo', change it so the sort is now 'highwater, foo'.