SPARK-2900. aggregate inputBytes per stage

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1826 from sryza/sandy-spark-2900 and squashes the following commits:

43f9091 [Sandy Ryza] SPARK-2900
(cherry picked from commit df652ea02a3e42d987419308ef14874300347373)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>