HDFS-4350. Merging change 1441819 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1441821 13f79535-47bb-0310-9956-ffa450edef68
Merge HDFS-4445. All BKJM ledgers are not checked while tailing, So failover will fail. Contributed Vinay.



git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1441936 13f79535-47bb-0310-9956-ffa450edef68