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.
Nitpick -- we might just want to add the word 'synchronized' before `Queue` here, because I don't think it will be obvious to non-Python people that a module simply called `Queue` will have that property.