[SPARK-1912] Lazily initialize buffers for local shuffle blocks.

This is a simplified fix for SPARK-1912.

Author: Reynold Xin <rxin@apache.org>

Closes #2179 from rxin/SPARK-1912 and squashes the following commits:

b2f0e9e [Reynold Xin] Fix unit tests.
a8eddfe [Reynold Xin] [SPARK-1912] Lazily initialize buffers for local shuffle blocks.
[SPARK-3279] Remove useless field variable in ApplicationMaster

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2177 from sarutak/SPARK-3279 and squashes the following commits:

2955edc [Kousuke Saruta] Removed useless field variable from ApplicationMaster
[SPARK-3307] [PySpark] Fix doc string of SparkContext.broadcast()

 remove invalid docs

Author: Davies Liu <davies.liu@gmail.com>

Closes #2202 from davies/keep and squashes the following commits:

aa3b44f [Davies Liu] remove invalid docs