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/trunk@1500580 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-218-trunk-gd