Intern unicode strings comming from external storage

According to my findings, this saves ~0.5Gb of memory during load of
current dataset. I used Pympler to get data on most used object types,
unicode was the top one (columns are type, instance count, total memory
used):

  unicode 8058199 894651428

With this change numbers are:

  unicode 1312689 303504132

and unicode went to the 2nd place (after set). No other significant
changes noted.

Change-Id: I6ae02824071c95e11636c6e4a2ec3699ec3c365f
Merge "Intern unicode strings comming from external storage"