Merge pull request #115 from aarondav/shuffle-fix

Eliminate extra memory usage when shuffle file consolidation is disabled

Otherwise, we see SPARK-946 even when shuffle file consolidation is disabled.
Fixing SPARK-946 is still forthcoming.

(cherry picked from commit d4df4749a8738628aa79802c4fb2805d02f47bb8)
Signed-off-by: Reynold Xin <rxin@apache.org>