Allow controlling number of splits in distinct().

Log message which records RDD origin

custom serializers or Kryo registrators can be loaded.

This adds tracking to determine the "origin" of an RDD. Origin is defined by
the boundary between the user's code and the spark code, during an RDD's
instantiation. It is meant to help users understand where a Spark RDD is
coming from in their code.

This patch also logs origin data when stages are submitted to the scheduler.

Finally, it adds a new log message to fix an inconsitency in the way that
dependent stages (those missing parents) and independent stages (those
without) are logged during submission.

Added MapPartitionsWithSplitRDD.

number of samples used for sorting

- takeSample was biased towards earlier partitions
- There were some range errors in takeSample
- SampledRDDs with replacement didn't produce appropriate counts
  across partitions (we took exactly frac of each one)

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