[SPARK-2661][bagel]unpersist old processed rdd

Unpersist useless rdd during bagel iteration to make full use of memory.

Author: Daoyuan <daoyuan.wang@intel.com>

Closes #1519 from adrian-wang/bagelunpersist and squashes the following commits:

182c9dd [Daoyuan] rename var nextUseless to lastRDD
87fd3a4 [Daoyuan] bagel unpersist old processed rdd