HADOOP-10240. Merging change r1559278 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1559280 13f79535-47bb-0310-9956-ffa450edef68
Update hadoop-common/CHANGES.txt for HADOOP-10112 in branch2.3

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1559287 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5784. reserve space in the edit log header and fsImage header for feature flag section (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1559295 13f79535-47bb-0310-9956-ffa450edef68