Update failedHighCnt rule official-stockfish#2063

Treat all threads the same as main thread and increment
failedHighCnt on fail highs. This makes the search try
again at lower depth.

@vondele suggested also changing the reset of failedHighCnt
when there is a fail low. Tests including this passed so the
branch has been updated to include both changes. failedHighCnt
is now handled exactly the same in helper threads and the main
thread. Thanks vondele :-)

STC @ 5+0.05 th 4 :
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 7769 W: 1704 L: 1557 D: 4508
http://tests.stockfishchess.org/tests/view/5c9f19520ebc5925cfffd2a1

LTC @ 20+0.2 th 8 :
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 37888 W: 5983 L: 5889 D: 26016
http://tests.stockfishchess.org/tests/view/5c9f57d10ebc5925cfffd696

Bench 3824325

src/search.cpp

@@ -404,17 +404,14 @@ void Thread::search() {
                   beta = (alpha + beta) / 2;
                   alpha = std::max(bestValue - delta, -VALUE_INFINITE);
 
+                  failedHighCnt = 0;
                   if (mainThread)
-                  {
-                      failedHighCnt = 0;
                       mainThread->stopOnPonderhit = false;
-                  }
               }
               else if (bestValue >= beta)
               {
                   beta = std::min(bestValue + delta, VALUE_INFINITE);
-                  if (mainThread)
-                      ++failedHighCnt;
+                  ++failedHighCnt;
               }
               else
                   break;