When istiod is unavailable portforwarding requests scale with namespaces

[nrfox]

This is a followup to #5669. That issue solves for number of requests scaling with pods. This issue is to cover requests scaling with number of selected namespaces.

Steps to reproduce:

1. Turn on debug logging for Kiali
2. Scale istiod to 0
3. Navigate to the overview page
4. Observe number of attempts to connect to istiod in logs increase with the number of namespaces. There are better ways to observe the connections but observing Kiali's connection logs is the easiest way.

[nrfox]

I'm fairly sure we can remove the refresh handler in the cache altogether if we move to polling istiod. We already recreate the cache any time there is a write operation so we don't need to worry about returning stale data after a write. I think it's perfectly acceptable to return slightly stale data from the debug endpoints if the registry changes in-between polling periods. proxy status is already using polling rather than fetching the data on demand.

[jshaughn]

I agree with @nrfox .

[nrfox]

I've poked at this a little bit and because the istio config is being returned directly from the registry rather than the kube api, when you do an update through the kube api you'll sometimes get back stale data since the registry may not be updated by the time the cache is refreshed. Ideally we'd return the istio config from the kube api rather than the registry so that we can be sure that the previous write has been read. That's a larger refactor though than I was hoping for.