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.
I mean - is there any conditional build mechanism that distinguishes between the test/dev builds and the 'real' ones ? If so, is the intentional packet loss and other functionalities part of the 'real' build as well ?