Fix inconsistent and incorrect log messages in shuffle read path
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Merge pull request #42 from pwendell/shuffle-read-perf

Fix inconsistent and incorrect log messages in shuffle read path

The user-facing messages generated by the CacheManager are currently wrong and somewhat misleading. This patch makes the messages more accurate. It also uses a consistent representation of the partition being fetched (`rdd_xx_yy`) so that it's easier for users to trace what is going on when reading logs.