Retire slavesPositions

Save the current active position in each Thread
instead of keeping a centralized array in struct
SplitPoint.

This allow to skip a memset() call at each split.

No functional change.

src/thread.h

@@ -75,7 +75,6 @@ struct SplitPoint {
 
   // Shared data
   Mutex mutex;
-  Position* slavesPositions[MAX_THREADS];
   volatile uint64_t slavesMask;
   volatile int64_t nodes;
   volatile Value alpha;
@@ -110,6 +109,7 @@ struct Thread {
   Material::Table materialTable;
   Endgames endgames;
   Pawns::Table pawnsTable;
+  Position* activePosition;
   size_t idx;
   int maxPly;
   Mutex mutex;