Sanity check research->techs_researched from savegame
We should sanity check research->techs_researched loaded from the savegame. We could also completely drop it from the savegame, and just use the recalculated value (which we need to do anyway when savegame value differs from it, i.e., when the sanity check fails).
This was inspired by debugging of #44594.
Reply To cazfi
We could also completely drop it from the savegame