We now have logic to fall back to by-value comparison, when the bottom
slot is not from the current reader.
But this is silly, because if the bottom slot is from a different
reader, it means the tie-break case is not possible (since the current
reader didn't have the bottom value), so when the incoming ord equals
the bottom ord we should always return x > 0.
I added a new random string sort test case to TestSort...
I also renamed DocValues.SortedSource.getByValue -> getOrdByValue and
cleaned up some whitespace.