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/maxscale/service.h

@@ -46,11 +46,19 @@ void service_free(SERVICE *service);
 /**
  * @brief Shut all services down
  *
- * Stops all services and calls the destroyInstance entry points for all routers
- * and filter. This should only be called once by the main shutdown code.
+ * Turns on the shutdown flag in each service. This should be done as
+ * part of the MaxScale shutdown.
  */
 void service_shutdown(void);
 
+/**
+ * @brief Destroy all service router and filter instances
+ *
+ * Calls the @c destroyInstance entry point of each service' router and
+ * filters. This should be done after all worker threads have exited.
+ */
+void service_destroy_instances(void);
+
 /**
  * @brief Launch all services
  *
@@ -116,4 +124,4 @@ bool  service_set_param_value(SERVICE* service, CONFIG_PARAMETER* param, char* v
 void       printService(SERVICE *service);
 void       printAllServices(void);
 
-MXS_END_DECLS
+MXS_END_DECLS