Replace Join and Split operations with "Change" (or whatever)

[adammck]

I started with a single reshard operation, which took an arbitrary number of adjacent ranges and splits as inputs, and output a new set of adjacent ranges. But it proved too hard to implement, because basically nothing worked back then. So I replaced it with separate Join/Split ops.

But now things do mostly work, and there's a bunch of pointless divergence between Join and Split, which are actually the same thing. Maybe the terminology should stay, because it's obvious what "Split a range" means, but the implementation should be re-unified and called something.