Tweaking the futility pruning formula

Huge credit goes also to candirufish,
as the idea was first tried by him, and then tuned by me at multiple phases.

Tweaking the futility pruning formula to be a bit more selective about when pruning is applied.
Adjust the value added to the static eval based on the bestValue relative to ss->staticEval. If bestValue is significantly lower, we add a larger value.

Passed STC:
LLR: 2.98 (-2.94,2.94) <0.00,2.00>
Total: 37120 W: 9590 L: 9266 D: 18264
Ptnml(0-2): 130, 4301, 9385, 4603, 141
https://tests.stockfishchess.org/tests/view/6544cf90136acbc573523247

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 49632 W: 12381 L: 12033 D: 25218
Ptnml(0-2): 30, 5429, 13549, 5779, 29
https://tests.stockfishchess.org/tests/view/65453bc1136acbc573523a3c

closes https://github.com/official-stockfish/Stockfish/pull/4861

bench: 1107118