Informer send notification after closed

[localvar]

As discussed in #79 (comment), it is possible for the current mesh informer implementation to send notifications after it is closed, this is unexpected and may cause bugs.

To fix this, we can add a wait operation in informer.Close to ensure all notifications are sent before the function returns, the change itself is simple but we need to check the existing code to ensure the wait does not cause a deadlock.

If the above solution is not possible, we can document the behavior and let client code handle the delayed notifications.