fanchat.py totally doesn't work at all

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Actually running sketches/fanchat.py produces confusingly broken behavior. I haven't conclusively traced everything to a single source, but I will enumerate some problems related to its dysfunction here:

• fan.In doesn't update outstanding aggregate pauses when drains are added or removed
• fan.Out doesn't update its list of drains even when a drain has told its fount to stopFlow
• some of the commands echo back to the client when they shouldn't
• the channels defaultdict can't construct Channel objects because the constructor takes an argument
• _TransportDrain.flowingFrom breaks when self.fount is not None
• python fanchat.py and then ^C results in a traceback since _OutFount.stopFlow is not idempotent

and probably others

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Fixed some of these in d937800

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_TransportDrain issue addressed by 18096a4

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the points in fanchat.py itself addressed in 04c639a

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#46 turns fanchat into a unit test (well okay more like an integration test) that illustrates the problem much more straightforwardly

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#46 now has a viable fix in it; get rid of the backpressure cycle, which is a bug in fanchat. Other bugs were discovered in the course of this and they should also be fixed, of course...