Scale child node futility pruning with previous move history.

Idea is to do more futility pruning if previous move has bad histories and less if it has good histories.

passed STC
https://tests.stockfishchess.org/tests/view/61e3757fbabab931824e0db7
LLR: 2.96 (-2.94,2.94) <0.00,2.50>
Total: 156816 W: 42282 L: 41777 D: 72757
Ptnml(0-2): 737, 17775, 40913, 18212, 771

passed LTC
https://tests.stockfishchess.org/tests/view/61e43496928632f7813a5535
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 349968 W: 94612 L: 93604 D: 161752
Ptnml(0-2): 300, 35934, 101550, 36858, 342

closes https://github.com/official-stockfish/Stockfish/pull/3903

bench 4720954

src/search.cpp

@@ -777,7 +777,8 @@ namespace {
     // The depth condition is important for mate finding.
     if (   !ss->ttPv
         &&  depth < 9
-        &&  eval - futility_margin(depth, improving) >= beta
+        &&  eval - futility_margin(depth, improving) - (ss-1)->statScore / 256 >= beta
+        &&  eval >= beta
         &&  eval < 15000) // 50% larger than VALUE_KNOWN_WIN, but smaller than TB wins.
         return eval;