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.
Most common usage of 'serialization' as a word, I know of, is to get an other representation of the the data at hand. Hence, while reading this at first my mind went to imagining method's code serialization somewhere.

If I understand correctly, the above is more about refusing to run the method's code mutiple times in parallel ?