Quiet move soft fail high bonus

Extra bonus for quiet move creating a huge soft fail high (triggered
in 21% of quiet bestmoves on a normal bench run). Pb00067 original idea
using PawnValueMg.

Passed STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 138207 W: 28060 L: 27295 D: 82852
http://tests.stockfishchess.org/tests/view/5b14471b0ebc5902a81689c1

Passe LTC:
LLR: 2.94 (-2.94,2.94) [0.00,5.00]
Total: 157289 W: 23200 L: 22518 D: 111571
http://tests.stockfishchess.org/tests/view/5b149dde0ebc5902a8b41c5a

bench: 4441320

src/search.cpp

@@ -1164,7 +1164,8 @@ namespace {
     {
         // Quiet best move: update move sorting heuristics
         if (!pos.capture_or_promotion(bestMove))
-            update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
+            update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, 
+							   stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
         else
             update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));