HDFS-5776 Support 'hedged' reads in DFSClient

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1571466 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5776 Support 'hedged' reads in DFSClient

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1571467 13f79535-47bb-0310-9956-ffa450edef68
YARN-1678. Fair scheduler gabs incessantly about reservations (Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1571468 13f79535-47bb-0310-9956-ffa450edef68