when invFunc is None, `reduceByKeyAndWindow(func, None, winsize, slidesize)` is equivalent to

     reduceByKey(func).window(winsize, slidesize).reduceByKey(winsize, slidesize)

and no checkpoint is necessary. The corresponding Scala code does exactly that, but Python code always creates a windowed stream with obligatory checkpointing. The patch fixes this.

I do not know how to unit-test this.

Author: David Tolpin <david.tolpin@gmail.com>

Closes #9888 from dtolpin/master.