FileWatcher polling fallback

Hi,

As a naif solution to the https://github.com/istio/istio/issues/10469 I have put in place a PR https://github.com/istio/istio/pull/11129. It just fallback to watch the files every second if setting the fsnotify watcher fails, maybe we need to put some time between each file so we don’t toast the server in case the files per directory is huge.

To unit test that just run same test as fsnotify but with this mechanism in the test suite.

What do you think?

BR

Quique