HDFS-6328. Clean up dead code in FSDirectory. Contributed by Haohui Mai.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1593755 13f79535-47bb-0310-9956-ffa450edef68
YARN-1987. Wrapper for leveldb DBIterator to aid in handling database exceptions. (Jason Lowe via kasha)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1593757 13f79535-47bb-0310-9956-ffa450edef68