Re-introduce "keep pawns on both flanks"

Re-introduce the "keep pawns on both flanks" idea.

STC yellow:
LLR: -2.95 (-2.94,2.94) [0.00,5.00]
Total: 93279 W: 20175 L: 19853 D: 53251
http://tests.stockfishchess.org/tests/view/5b8a00370ebc592cf274916a

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 11440 W: 1960 L: 1792 D: 7688
http://tests.stockfishchess.org/tests/view/5b8a329f0ebc592cf2749615

Closes https://github.com/official-stockfish/Stockfish/pull/1761

Bench: 4609645