expose byte_buffer in grpc

gistfile1.txt

	
  /// Is this ByteBuffer valid?
  bool Valid() const { return (buffer_ != nullptr); }

+  grpc_byte_buffer* c_buffer() { return buffer_; }
 private:
  friend class SerializationTraits<ByteBuffer, void>;
  friend class ServerInterface;
Context not available.
    buffer_ = buf;
  }

-  grpc_byte_buffer* c_buffer() { return buffer_; }
  grpc_byte_buffer** c_buffer_ptr() { return &buffer_; }

  class ByteBufferPointer {