HDFS-5364. Add OpenFileCtx cache. Contributed by Brandon Li

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1539834 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5326. add modifyDirective to cacheAdmin (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1539839 13f79535-47bb-0310-9956-ffa450edef68