Fix races in gensfen as detected with thread sanitizer.

RootInTB was an incorrectly shared global, probably leading to wrong scoreing

Minor:
 setting TB global state from input by all threads (all threads write same values)
 setting Limits global state by all threads (idem)
 thread counting for finalization

CI can be enabled once races are fixed in the learner, manually goes like:
```
make clean && make -j2 ARCH=x86-64-modern sanitize=thread    optimize=no debug=yes build
../tests/instrumented_learn.sh --sanitizer-thread
```

Needs some review.

src/learn/multi_think.h

@@ -96,10 +96,7 @@ private:
 	std::mutex loop_mutex;
 
 	// Thread end flag.
-	// vector<bool> may not be reflected properly when trying to rewrite from multiple threads...
-	typedef uint8_t Flag;
-	std::vector<Flag> thread_finished;
-
+        std::atomic<uint64_t> threads_finished;
 };
 
 // Mechanism to process task during idle time.