MAPREDUCE-5604. Merging change r1538035 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1538038 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5037. Active NN should trigger its own edit log rolls. Contributed by Andrew Wang.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1538061 13f79535-47bb-0310-9956-ffa450edef68