svn merge -c 1532932 from trunk for HDFS-4376. Fix race conditions in Balancer.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1532933 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5360. Improvement of usage message of renameSnapshot and deleteSnapshot. Contributed by Shinichi Yamashita.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1532938 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5334. Merge change r1532949 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1532950 13f79535-47bb-0310-9956-ffa450edef68