Improve move order near the root

Current move histories are known to work well near the leaves, whilst at
higher depths they aren't very helpful. To address this problem this
patch introduces a table dedicated for what's happening at plies 0-3.
It's structured like mainHistory with ply index instead of color.
It get cleared with each new search and is filled during iterative
deepening at higher depths when recording successful quiet moves near
the root or traversing nodes which were in the principal variation
(ttPv).

Medium TC (20+0.2):
https://tests.stockfishchess.org/tests/view/5e4d358790a0a02810d096dc
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 100910 W: 16682 L: 16376 D: 67852
Ptnml(0-2): 1177, 10983, 25883, 11181, 1231

LTC:
https://tests.stockfishchess.org/tests/view/5e4e2cb790a0a02810d09714
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 80444 W: 10495 L: 10095 D: 59854
Ptnml(0-2): 551, 7479, 23803, 7797, 592

closes https://github.com/official-stockfish/Stockfish/pull/2557

Bench: 4705960

src/movepick.cpp

@@ -56,10 +56,10 @@ namespace {
 /// ordering is at the current node.
 
 /// MovePicker constructor for the main search
-MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,
-                       const CapturePieceToHistory* cph, const PieceToHistory** ch, Move cm, Move* killers)
-           : pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch),
-             refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}}, depth(d) {
+MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh, const LowPlyHistory* lp,
+                       const CapturePieceToHistory* cph, const PieceToHistory** ch, Move cm, Move* killers, int pl)
+           : pos(p), mainHistory(mh), lowPlyHistory(lp), captureHistory(cph), continuationHistory(ch),
+             refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}}, depth(d) , ply(pl) {
 
   assert(d > 0);
 
@@ -115,7 +115,8 @@ void MovePicker::score() {
                    + 2 * (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
                    + 2 * (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
                    + 2 * (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]
-                   +     (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)];
+                   +     (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)]
+                   + (ply < MAX_LPH ?  4 * (*lowPlyHistory)[ply][from_to(m)] : 0);
 
       else // Type == EVASIONS
       {