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.
15 ight be an intimidating number of messages, it might be cool to have a table here listing all of them. This would allow for the reader to see that these are grouped somehow and possibly the reader would take a break here to think how the system might be working, just by looking at this table.