YARN-1730. Implemented simple write-locking in the LevelDB based timeline-store. Contributed by Billie Rinaldi.
svn merge --ignore-ancestry -c 1574145 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1574147 13f79535-47bb-0310-9956-ffa450edef68