backpressure policy drain/fount for "Drop"

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Sometimes, when encountering backpressure – assuming that the output type of the given fount is a discrete message and not part of a continuous stream like IFragment – the thing the fount actually wants to do instead of delivering things is to drop them. For example, in the case of a fan.Out keeping a bunch of observers up to date about the current state of something, you may want to allow some of those observers to just not see messages while their respective buffers are full.

See #35