svn merge -c 1423189 from trunk for HADOOP-9140 Cleanup rpc PB protos.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1483137 13f79535-47bb-0310-9956-ffa450edef68
Merged in HADOOP-9218 Document the Rpc-wrappers used internally (sanjay Radia)


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Merged in HADOOP-9163


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Merged HADOOP-9151


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Merged HADOOP-9380


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Merged HADOOP-9425


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Merge remote branch 'origin/branch-2' into branch-2-gd