Preparing to release hadoop-2.3.0

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1567111 13f79535-47bb-0310-9956-ffa450edef68
Release notes for hadoop-2.3.0.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1567118 13f79535-47bb-0310-9956-ffa450edef68
Merge trunk to HDFS-4685.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4685@1567238 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'HDFS-4685' into HDFS-5925
HDFS-5925: code review feedback