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.
It might be cool to mention how this influences testing here. E.g. that you could have a build system that would add different class versions depending whether this is a final package or the one used for testing. And how is that different from this approach here, e.g. that the approach above might allow for tools like Chaos Monkey to run in production as well and what are the benefits here. [Thanks to @pshemass for the CM suggestion!]