RouteDefinitionLocator features

[libinglong]

In this issue, I faced the same problem.RouteDefinitionLocator.getRouteDefinitions gets invoked every 30 seconds because nacos(a service discovery component) send HeartbeatEvent which causes RefreshRoutesEvent.

Can we add a feature to determine whether should update some routedefinitions. In my application, there are routes store in mysql. A service discovery update event should not update routes in mysql.