Call destroyInstance after workers have exited

Router and filter instances cannot be destroyed before all worker
threads have exited. Otherwise there is a risk that data gets deleted
while someone might still be accessing it. Further, since all router
and filter instances are created by the main-thread it is better that
they are deleted by that thread as well (and not by whichever thread
happens to execute service_shutdown()). That will reduce the risk that
some unknown assumptions are violated.

server/core/gateway.cc

@@ -2044,6 +2044,12 @@ int main(int argc, char **argv)
     {
         thread_wait(threads[thread_id]);
     }
+
+    /*<
+     * Destroy the router and filter instances of all services.
+     */
+    service_destroy_instances();
+
     /*<
      * Wait the flush thread.
      */