[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.