[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
Ahh, gotcha, apologies. Any chance we could get a new unit test for the particular problem in this PR? The changes you added do look correct and verifiable from just reading them, so I'd be fine with this being merged without, but doesn't hurt to ask :)