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.
How do I access this  service ? Do I go to one of the servers or is there an explicit gateway to the system ? A very quick run on the questions above might help people to form the image of the system a a whole.
Also, maybe a simple figure to depict the whole system might help.