Fix uninitialized memory usage

After increasing the number of threads, the histories were not cleared,
resulting in uninitialized memory usage.

This patch fixes this by clearing threads histories in Thread c'tor as
is the idomatic way.

This fixes issue 1227

No functional change.

src/thread.h

@@ -52,6 +52,7 @@ public:
   explicit Thread(size_t);
   virtual ~Thread();
   virtual void search();
+  void clear();
   void idle_loop();
   void start_searching();
   void wait_for_search_finished();