Refine statscore for captures

Continuation of previous attempts there.
Now instead of using capture history with a static offset also add the
value of the captured piece in the same way at it is used in movepicker.

Passed STC:
https://tests.stockfishchess.org/tests/view/674aa3d386d5ee47d95404aa
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 116480 W: 30433 L: 29999 D: 56048
Ptnml(0-2): 361, 13720, 29662, 14118, 379

Passed LTC:
https://tests.stockfishchess.org/tests/view/674c4b2d86d5ee47d954073f
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 133542 W: 34365 L: 33847 D: 65330
Ptnml(0-2): 78, 14585, 36934, 15089, 85

closes https://github.com/official-stockfish/Stockfish/pull/5706

Bench: 934447

src/search.cpp

@@ -1180,7 +1180,10 @@ moves_loop:  // When in check, search starts here
             r -= 1879;
 
         if (capture)
-            ss->statScore = 0;
+            ss->statScore =
+              7 * int(PieceValue[pos.captured_piece()])
+              + thisThread->captureHistory[movedPiece][move.to_sq()][type_of(pos.captured_piece())]
+              - 5000;
         else
             ss->statScore = 2 * thisThread->mainHistory[us][move.from_to()]
                           + (*contHist[0])[movedPiece][move.to_sq()]