HDFS-5203. Concurrent clients that add a cache directive on the same path may prematurely uncache each other.  (Chris Nauroth via Colin Patrick McCabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4949@1533651 13f79535-47bb-0310-9956-ffa450edef68