SPARK-1076: [Fix #578] add @transient to some vals

I'll try to be more careful next time.

Author: Xiangrui Meng <meng@databricks.com>

Closes #591 and squashes the following commits:

2b4f044 [Xiangrui Meng] add @transient to prev in ZippedWithIndexRDD add @transient to seed in PartitionwiseSampledRDD