Replace easyMove with simple scheme

Reduces time for a stable bestMove, giving some of the won time for the next move.

the version before the pvDraw passed both STC and LTC

passed STC:
http://tests.stockfishchess.org/tests/view/59e98d5a0ebc590ccbb896ec
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 78561 W: 13945 L: 13921 D: 50695
elo =    0.106 +-    1.445 LOS:   55.716%

passed LTC:
http://tests.stockfishchess.org/tests/view/59eb9df90ebc590ccbb897ae
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 29056 W: 3640 L: 3530 D: 21886
elo =    1.315 +-    1.982 LOS:   90.314%

This version, rebased on pvDrawPR with the obvious change, was verified again on STC:

http://tests.stockfishchess.org/tests/view/59ee104e0ebc590ccbb89899
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 19890 W: 3648 L: 3525 D: 12717
elo =    2.149 +-    2.895 LOS:   92.692%

and LTC:
http://tests.stockfishchess.org/tests/view/59f9673a0ebc590ccbb89ea0
Total             :    17966
Win               :     2273 (  12.652%)
Loss              :     2149 (  11.961%)
Draw              :    13544 (  75.387%)
Score             :   50.345%
Sensitivity       :    0.014%
2*(W-L)/(W+L)     :    5.608%

LLR  [-3.0,  1.0] :     2.95

BayesElo range    : [  -1.161,   4.876,  10.830] (DrawElo:  341.132)
LogisticElo range : [  -0.501,   2.105,   4.677]
LOS               :   94.369 %

LTC again:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 17966 W: 2273 L: 2149 D: 13544
LogisticElo range : [ -0.501, 2.105, 4.677]
LOS : 94.369 %

unchanged bench: 5234652

src/thread.h

@@ -82,8 +82,8 @@ struct MainThread : public Thread {
   void search() override;
   void check_time();
 
-  bool easyMovePlayed, failedLow;
-  double bestMoveChanges;
+  bool failedLow;
+  double bestMoveChanges, previousTimeReduction;
   Value previousScore;
   int callsCnt;
 };