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 itneresting to see if the debugging section can be moved to the very beginning. To start the code description with the setup of the test and debugging environment, to show that this is very helpful during the development and only then start to describe the system that has been built using this. What do you think ?