HDFS-4909. Avoid protocol buffer RPC namespace clashes (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1588093 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10499. Merging change r1588098 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1588099 13f79535-47bb-0310-9956-ffa450edef68