[SPARK-2503] Lower shuffle output buffer (spark.shuffle.file.buffer.kb) to 32KB.

This can substantially reduce memory usage during shuffle.

Author: Reynold Xin <rxin@apache.org>

Closes #1781 from rxin/SPARK-2503-spark.shuffle.file.buffer.kb and squashes the following commits:

104b8d8 [Reynold Xin] [SPARK-2503] Lower shuffle output buffer (spark.shuffle.file.buffer.kb) to 32KB.