[SPARK-2571] Correctly report shuffle read metrics.

Currently, shuffle read metrics are incorrectly reported when stages have multiple shuffle dependencies (they are set to be the metrics from just one of the shuffle dependencies, rather than the accumulated metrics from all of the shuffle dependencies).  This fixes that problem, and should probably be back-ported to the 0.9 branch.

Thanks @ryanra for discovering this problem!

cc @rxin @andrewor14
It expects 2000 bytes read because the listener is adding up the total across all tasks in the stage -- so 1000 for the earlier task + 1000 for this task.  The lines that I deleted were redundant (remoteBytesRead was already set to 1000 and shuffleReadMetrics had already been set as taskMetrics.shuffleReadMetrics).