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.
At first I thought that it might be beneficial to explain what a network partition event is explicitely here, but I think that  the two sentences above introduce terminology implicitely so it can slide into reader's brain via a backdoor. Nice work !