Replace deprecated SQLContext with SparkSession for DataFrame / DataSet / RDD generators

[akreit]

E.g. the DataframeGenerator.arbitraryDataFrame() method takes a SQLContext as parameter. According to the Spark docs [1], this has been deprecated since Spark 2.0 in favor of SparkSession. Would be nice to replace this.

[1] https://spark.apache.org/docs/latest/api/java/org/apache/spark/sql/SQLContext.html

[holdenk]

Good call, since we don't support 1.X and will probably drop 2.4 before too long this seems like a good cleanup issue. Ideally we'd have both entry points for awhile for code compatability.