_flush(callback) {
        if (!this.streamClosed) {
            this.log.error('stream ended without closing chunked encoding');
            return callback(errors.InvalidArgument);
        }
        return callback();
    }
This function is executed when there is no more data to be read but before the stream is closed
We will verify that the trailing checksum structure was upheld

@param {function} callback - Callback(err, data)
@return {function} executes callback with err if applicable