When composing ("chaining") multiple windowing operations on the same
stream are watermarks transmitted down stream after window evaluation,
and are the records emitted from WindowFunctions given timestamps? Do I
need to or should I always assignTimestampsAndWatermarks to the
outputsof window evaluations if I want to window again? If automatically
assigned, how should I think about them in an event time context? Would
the event time of a record resulting from a WindowFunction be the
window's end time in the case of a TimeWindow?
