HADOOP-9944. Fix RpcRequestHeaderProto.callId to be sint32 rather than uint32 since ipc.Client.CONNECTION_CONTEXT_CALL_ID is signed (i.e. -3). Contributed by Arun C. Murthy.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1523885 13f79535-47bb-0310-9956-ffa450edef68