Use fixed "dummy id" rather than generating a new one each time

If you do 

    LoadByUniqueConstraint<Foo>(arrayOfIds);

    foreach(var id in arrayOfIds) {
        LoadByUniqueConstraint<Foo>(id);
    }

You get a request for every Foo that does not exist.  Using a Guid as the "id" in this case is a useful hack that avoids having to line up 2 arrays, but there is no need to generate a new Guid each time.  Instead I've just hardcoded a fixed Guid and we'll use this every time.