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.
It might be cool to have some glues sentences here or in the end of the section above to stick those two together. E.g. tell about how the problems listed above led you to the implementation of the library or how does the library help/solve the ones above