This writer implements the core logic of time-window based data
segregation. It splits the fragment stream provided by a reader, such
that each atom (cell) in the stream will be written into a consumer
based on the time-window its timestamp belongs to. The end result is
that each consumer will only see fragments, whoose atoms all have
timestamps belonging to the same time-window.
When a mutation fragment has atoms belonging to different time-windows,
it is split into as many fragments as needed so each has only atoms
that belong to the same time-window.
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