Convert init of eval to async option

So to be done only once at startup and in the (unlikely)
cases that a relevant UCI parameter is changed, instead
of doing it at the beginning of each search.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/ucioption.h

@@ -57,7 +57,7 @@ private:
   std::string defaultValue, currentValue, type;
   int min, max;
   size_t idx;
-  Fn* on_change_action;
+  Fn* on_change;
 };