HDFS-5230. Introduce RpcInfo to decouple XDR classes from the RPC API. Contributed by Haohui Mai

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1527726 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5279: code review feedback
Merge branch 'trunk' into HDFS-5279