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.
This is a neat tradeoff to discuss; at what point do you 'give up' on trying to guess what the consumer of your library/tool is going to want, and instead just give them a hook to implement some custom behaviour?