Make WindowMappingFn#maximumLookback Configurable but Final

[tgroh]

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This enforces that it return a constant value.

[tgroh]

R: @bjchambers

[coveralls]

Changes Unknown when pulling d9092c9 on tgroh:window_mapping_fun_params into ** on apache:master**.

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[bjchambers]

Should there be a test or something that ensures that the return value never exceeds the maximum lookback?

[tgroh]

That should likely be a checkState or similar within the runner; it doesn't seem meaningfully possible to assert that via test.