svn merge -c 1476399 from trunk for HDFS-4721. Speed up lease recovery by avoiding stale datanodes and choosing he datanode with the most recent heartbeat as the primary.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1476400 13f79535-47bb-0310-9956-ffa450edef68