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/thread.h

@@ -61,11 +61,12 @@ public:
   Endgames endgames;
   size_t idx, PVIdx;
   int maxPly, callsCnt;
-  uint64_t tbHits;
+  std::atomic<uint64_t> tbHits;
+  std::atomic<uint64_t> nodes;
 
   Position rootPos;
   Search::RootMoves rootMoves;
-  Depth rootDepth;
+  std::atomic<Depth> rootDepth;
   Depth completedDepth;
   std::atomic_bool resetCalls;
   CounterMoveStat counterMoves;