Rewrite time formula

Time management is really too complex, our aim is
to simplify it, but for time being at least rewrite
in an understandable way.

No functional change.

src/thread.h

@@ -80,7 +80,7 @@ struct MainThread : public Thread {
 
   bool easyMovePlayed, failedLow;
   double bestMoveChanges;
-  Value previousMoveScore;
+  Value previousScore;
 };