HDFS-4270. Replications of the highest priority should be allowed to choose a source datanode that has reached its max replication limit (Derek Dagit via tgraves)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1428883 13f79535-47bb-0310-9956-ffa450edef68