[SPARK-7889] [Core] WiP History Caching: xu tingjun's patch against master#9913

steveloughran wants to merge 2 commits intoapache:masterapache/spark:masterfrom

steveloughran:history/SPARK-7889-XuTingjunsteveloughran/spark:history/SPARK-7889-XuTingjunCopy head branch name to clipboard