Title: Production usable mechanism for waiting for listeners ready

Description:
When building an application with envoy, it is useful to know when the newly configured envoy is ready to server requests, so that an race free signal may be given to consumers that connections to the newly started application should be expected to succeed. Such a mechanism already exists in envoy, but it's name (TestHooks) implies that it shouldn't be used in production code. Despite that, it is wired into production code and provides a reliable mechanism to signal when listeners are active and ready to listen.

That class name should be changed to ListenerHooks to properly signal that it may be used in production code.