svn merge -c 1428739 from trunk for HDFS-4270. Introduce soft and hard limits for max replication so that replications of the highest priority are allowed to choose a source datanode that has reached its soft limit but not the hard limit.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1428743 13f79535-47bb-0310-9956-ffa450edef68