HDFS-3970. Fix bug causing rollback of HDFS upgrade to result in bad VERSION file. Contributed by Vinay and Andrew Wang.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1541939 13f79535-47bb-0310-9956-ffa450edef68
HDFS-4995. Make getContentSummary less expensive. Contributed by Kihwal Lee.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1541978 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-0.23' into branch-0.23-gd
Merge remote branch 'origin/branch-0.23-gd' into fb-HADOOP-9291-branch-0.23-gd