private void doPreambleResponse(Message resp) throws IOException {
    ResponseHeader header = ResponseHeader.newBuilder().setCallId(-1).build();
    ByteBuffer buf = ServerCall.createHeaderAndMessageBytes(resp, header, 0, null);
    BufferChain bufChain = new BufferChain(buf);
    doRespond(() -> bufChain);
  }
Has the request header and the request param and optionally encoded data buffer all in this one
array.
<p/>
Will be overridden in tests.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/ServerRpcConnection.java