pre-allocate rows in BackEndDataTable to make it fast!

[asiripanich]

This might be better if we want to add new rows using a loop and data.table::set.

• https://stackoverflow.com/questions/16792001/add-a-row-by-reference-at-the-end-of-a-data-table-object
• https://stackoverflow.com/questions/20689650/how-to-append-rows-to-an-r-data-frame
• http://www.win-vector.com/blog/2015/07/efficient-accumulation-in-r/