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 wasn't very clear, sorry. I meant that the end reader of the chapter might be interested in history of the names, laid out in a sentence or two, as this might be a fun read. I don't really recall the origins of the names here so I thought that you, @djmitche, might dig them up and add to the chapter.