HDFS-6357. SetXattr should persist rpcIDs for handling retrycache with Namenode restart and HA. Contributed by Uma Maheswara Rao G.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2006@1593717 13f79535-47bb-0310-9956-ffa450edef68