Method AddFirstLine

catboost/libs/data/async_row_processor.h:39–43

sometimes we need to separately process first data, but add it to usual processing as well

        // sometimes we need to separately process first data, but add it to usual processing as well
        void AddFirstLine(TData&& firstLine) {
            CB_ENSURE(!FirstLineInReadBuffer, "TAsyncRowProcessor: double call to AddFirstLine");
            ReadBuffer[0] = std::move(firstLine);
            FirstLineInReadBuffer = true;
        }

        /*
         * readFunc should be of type 'bool(TData* data)',