HDFS-6328. Merge r1593755 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1593756 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/branches/branch-2@1593758 13f79535-47bb-0310-9956-ffa450edef68