First draft of cluster chapter

I'm reasonably happy with the shape and content of the chapter here, so I think it's ready for a review.

The code is mostly in place, although there are a number of "TODO" tags that I'm still pondering.  The big one -- that affects the chapter -- is to remove the 'catchup' support entirely.  I can't quite convince myself that the implementation will still perform correctly without it, though.
That might be really hard to read: "here's some discussion of tests for code you don't even know about"

It's worth thinking about *talking* about tests earlier, and you commented above about the perfect place for it: where I introduce the component model.