`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

Author: Xiangrui Meng <meng@databricks.com>

Closes #3264 from mengxr/SPARK-4398 and squashes the following commits:

8953c41 [Xiangrui Meng] follow davies' suggestion
cbd58e3 [Xiangrui Meng] specialize sc.parallelize(xrange)