Rate limiting concerns about the resource collectors

[jshaofuturewei]

Resource collectors do not have rate limiting to list all the site resources as shown here

https://github.com/futurewei-cloud/global-resource-scheduler/blob/master/globalscheduler/pkg/scheduler/client/informers/siteresources/siteresources.go#L89

It happens to other components as well when facing heavy loads of requests.

The concerns is triggered when we create 1000 cluster sites.