Double weight of capture history

We double in this patch the weight of the capture history table in the
local scoring of captures for move ordering.

The capture history table is indexed by the triplet (capturing piece,
capture square, captured piece) and gets information like "it seems to
have been historically good in that part of the search tree to capture
a pawn with a rook on g3, even if it seems to lose material", and affect
the normaly pure « Most Valuable Victim » ordering of captures.

Finished yellow at STC after 228842 games (posting a +1.36 Elo gain):
LLR: -2.95 (-2.94,2.94) [0.00,4.00]
Total: 228842 W: 50894 L: 50152 D: 127796
http://tests.stockfishchess.org/tests/view/5b714bb00ebc5902bdba332d

Passed LTC:
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 43251 W: 7425 L: 7131 D: 28695
http://tests.stockfishchess.org/tests/view/5b71c7d40ebc5902bdba3e51

Thanks to user Vizvezdenec for running the LTC test.

Closes #1736

Bench: 4272361

src/movepick.cpp

@@ -111,7 +111,7 @@ void MovePicker::score() {
   for (auto& m : *this)
       if (Type == CAPTURES)
           m.value =  PieceValue[MG][pos.piece_on(to_sq(m))]
-                   + (*captureHistory)[pos.moved_piece(m)][to_sq(m)][type_of(pos.piece_on(to_sq(m)))] / 16;
+                   + (*captureHistory)[pos.moved_piece(m)][to_sq(m)][type_of(pos.piece_on(to_sq(m)))] / 8;
 
       else if (Type == QUIETS)
           m.value =  (*mainHistory)[pos.side_to_move()][from_to(m)]