client-go: support waiting for SharedInformerFactory shutdown
client-go: support waiting for SharedInformerFactory shutdown

SharedInformerFactory starts goroutines in Start and those can be stopped by
closing the stop channel. However, there was no API that waits for the
goroutines.

This is a problem for unit testing. A test has to return while the informers
are still running, which may get flagged by tools like
https://github.com/uber-go/goleak or by klog/ktesting when those informers
lead to log output.

While at it, more documentation gets added to address
https://github.com/kubernetes/kubernetes/issues/65036.

Kubernetes-commit: e89d1d47e8365cad31600b17dab662d3c8a359dd

kubernetes/kube-aggregator,kubernetes/kube-aggregator