RPC Client performance improvement

  The current implementation copies the rpcRequest and header to a ByteArrayOutputStream in order to calculate the total length of the sent request, and then writes it to the socket buffer.

  But if the rpc engine is ProtobufRpcEngine2, we can pre-calculate the request size, and then send the request directly to the socket buffer, reducing a memory copy. And avoid allocating 1024 bytes of ResponseBuffer each time a request is sent.