Follow up to #5515 for #5507

More specifically, the union of the two lists of top 10 ids when gc() is before and after ps_mem().

Keep repeating until we see, hopefully, the plot ending closer to the starting RAM usage. Currently it ends at 265MB. Perhaps with #5516 too it should be possible to end comfortably under 100MB. Since R itself plus data.table loaded uses 67MB.