Refactor root history into low ply history

This patch changes root history to low ply history - butterfly history for plies < 4.
Doubles weight of this history for root, latter plies have lesser effect.

Passed STC:
https://tests.stockfishchess.org/tests/view/66f77d2386d5ee47d953b65d
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 180992 W: 47362 L: 46830 D: 86800
Ptnml(0-2): 554, 21499, 45928, 21891, 624

Passed LTC:
https://tests.stockfishchess.org/tests/view/66fb557986d5ee47d953b8e5
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 42462 W: 11013 L: 10682 D: 20767
Ptnml(0-2): 33, 4518, 11795, 4855, 30

closes https://github.com/official-stockfish/Stockfish/pull/5614

Bench 1264335

src/movepick.cpp

@@ -82,20 +82,20 @@ MovePicker::MovePicker(const Position&              p,
                        Move                         ttm,
                        Depth                        d,
                        const ButterflyHistory*      mh,
-                       const ButterflyHistory*      rh,
+                       const LowPlyHistory*         lph,
                        const CapturePieceToHistory* cph,
                        const PieceToHistory**       ch,
                        const PawnHistory*           ph,
-                       bool                         rn) :
+                       int                          pl) :
     pos(p),
     mainHistory(mh),
-    rootHistory(rh),
+    lowPlyHistory(lph),
     captureHistory(cph),
     continuationHistory(ch),
     pawnHistory(ph),
     ttMove(ttm),
     depth(d),
-    rootNode(rn) {
+    ply(pl) {
 
     if (pos.checkers())
         stage = EVASION_TT + !(ttm && pos.pseudo_legal(ttm));
@@ -179,8 +179,8 @@ void MovePicker::score() {
                         : pt == ROOK && bool(to & threatenedByMinor) ? 24335
                                                                      : 0);
 
-            if (rootNode)
-                m.value += 4 * (*rootHistory)[pos.side_to_move()][m.from_to()];
+            if (ply < 4)
+                m.value += 8 * (*lowPlyHistory)[ply][m.from_to()] / (1 + 2 * ply);
         }
 
         else  // Type == EVASIONS