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.

(cherry picked from commit ea34c521025d3408d44d45ab5c132fd9791794f6)
Signed-off-by: Reynold Xin <rxin@apache.org>