Fix four data races.

the nodes, tbHits, rootDepth and lastInfoTime variables are read by multiple threads, but not declared atomic, leading to data races as found by -fsanitize=thread. This patch fixes this issue. It is based on top of the CI-threading branch (PR #1129), and should fix the corresponding CI error messages.

The patch passed an STC check for no regression:

http://tests.stockfishchess.org/tests/view/5925d5590ebc59035df34b9f
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 169597 W: 29938 L: 30066 D: 109593

Whereas rootDepth and lastInfoTime are not performance critical, nodes and tbHits are. Indeed, an earlier version using relaxed atomic updates on the latter two variables failed STC testing (http://tests.stockfishchess.org/tests/view/592001700ebc59035df34924), which can be shown to be due to x86-32 (http://tests.stockfishchess.org/tests/view/592330ac0ebc59035df34a89). Indeed, the latter have no instruction to atomically update a 64bit variable. The proposed solution thus uses a variable in Position that is accessed only by one thread, which is copied every few thousand nodes to the shared variable in Thread.

No functional change.

Closes #1130
Closes #1129

src/position.h

@@ -134,6 +134,8 @@ public:
   void undo_move(Move m);
   void do_null_move(StateInfo& newSt);
   void undo_null_move();
+  void increment_nodes();
+  void increment_tbHits();
 
   // Static Exchange Evaluation
   bool see_ge(Move m, Value value = VALUE_ZERO) const;
@@ -151,6 +153,7 @@ public:
   bool is_chess960() const;
   Thread* this_thread() const;
   uint64_t nodes_searched() const;
+  uint64_t tb_hits() const;
   bool is_draw(int ply) const;
   int rule50_count() const;
   Score psq_score() const;
@@ -185,6 +188,7 @@ private:
   Square castlingRookSquare[CASTLING_RIGHT_NB];
   Bitboard castlingPath[CASTLING_RIGHT_NB];
   uint64_t nodes;
+  uint64_t tbHits;
   int gamePly;
   Color sideToMove;
   Thread* thisThread;
@@ -353,6 +357,18 @@ inline uint64_t Position::nodes_searched() const {
   return nodes;
 }
 
+inline void Position::increment_nodes() {
+  nodes++;
+}
+
+inline uint64_t Position::tb_hits() const {
+  return tbHits;
+}
+
+inline void Position::increment_tbHits() {
+  tbHits++;
+}
+
 inline bool Position::opposite_bishops() const {
   return   pieceCount[W_BISHOP] == 1
         && pieceCount[B_BISHOP] == 1