[SPARK-4398][PySpark] specialize sc.parallelize(xrange)

`sc.parallelize(range(1 << 20), 1).count()` may take 15 seconds to finish and the rdd object stores the entire list, making task size very large. This PR adds a specialized version for xrange.

@JoshRosen @davies
How about pre-calculate all the boundaries for all the partitions?