[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
Gotcha, no, that's fine. Funny that we had the test already but with relaxed constraints.