Tweak time management (failing eval)

Adjust fallingEval with score change in last 5 iterations. FallingEval adjusts
the time used on a move depending on whether the position score is better or
worse than on the previous move. This change adds a dependency on the score
change in the last 5 iterations of the current search.

Tests with original code:
STC :
LLR: 2.97 (-2.94,2.94) [-1.50,4.50]
Total: 18728 W: 4170 L: 4005 D: 10553
https://tests.stockfishchess.org/tests/view/5de68a5bb407ee7bfda68a94

LTC :
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 180217 W: 29214 L: 28551 D: 122452
https://tests.stockfishchess.org/tests/view/5de690a4b407ee7bfda68a9a

Revised code using a simple array instead of a deque and different values
gave a slightly quicker pass at LTC. The merged patch now uses this:

STC :
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 18616 W: 4114 L: 3950 D: 10552
https://tests.stockfishchess.org/tests/view/5debb790b7bdefd50db28d14

LTC :
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 134151 W: 21729 L: 21191 D: 91231
https://tests.stockfishchess.org/tests/view/5debc13fb7bdefd50db28d19

No functional change

src/thread.h

@@ -88,6 +88,7 @@ struct MainThread : public Thread {
 
   double previousTimeReduction;
   Value previousScore;
+  Value iterValue[4];
   int callsCnt;
   bool stopOnPonderhit;
   std::atomic_bool ponder;