Applies a reduce function to the window. The window function is called for each evaluation
of the window for each key individually. The output of the reduce function is interpreted as
a regular non-windowed stream.
This window will try and incrementally aggregate data as much as the window policies
permit. For example, tumbling time windows can aggregate the data, meaning that only one
element per key is stored. Sliding time windows will aggregate on the granularity of the
slide interval, so a few elements are stored per key (one per slide interval). Custom
windows may not be able to incrementally aggregate, or may need to store extra values in an