HDFS-4645.  Move from randomly generated block ID to sequentially generated block ID.  Contributed by Arpit Agarwal


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1501993 13f79535-47bb-0310-9956-ffa450edef68
In CHANGES.txt, move HDFS-4908 to 2.1.0-beta.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502010 13f79535-47bb-0310-9956-ffa450edef68