Move stop signal to Threads

Instead of having Signals in the search namespace,
make the stop variables part of the Threads structure.
This moves more of the shared (atomic) variables towards
the thread-related structures, making their role more clear.

No functional change

Closes #1149

src/thread.h

@@ -101,6 +101,8 @@ struct ThreadPool : public std::vector<Thread*> {
   uint64_t nodes_searched() const;
   uint64_t tb_hits() const;
 
+  std::atomic_bool stop, stopOnPonderhit;
+
 private:
   StateListPtr setupStates;
 };