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.
When does this happen? On every single operation that would require synchronization? Can you give an example of such an operation?