SPARK-2565. Update ShuffleReadMetrics as blocks are fetched

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1507 from sryza/sandy-spark-2565 and squashes the following commits:

74dad41 [Sandy Ryza] SPARK-2565. Update ShuffleReadMetrics as blocks are fetched
(cherry picked from commit 4c51098f320f164eb66f92ff0f26b0b595a58f38)

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