Lifecycle bug in WASM filter

[yanavlasov]

The shutdownExit envent handler should be held until at least when the registered callback is called:

fyi, this function may has a bug:

void Wasm::initializeLifecycle(Server::ServerLifecycleNotifier& lifecycle_notifier) {
  auto weak = std::weak_ptr<Wasm>(std::static_pointer_cast<Wasm>(shared_from_this()));
  lifecycle_notifier.registerCallback(Server::ServerLifecycleNotifier::Stage::ShutdownExit,
                                      [this, weak](Event::PostCb post_cb) {
                                        auto lock = weak.lock();
                                        if (lock) { // See if we are still alive.
                                          server_shutdown_post_cb_ = std::move(post_cb);
                                        }
                                      });
}

because the comment of registerCallback says the caller should keep the return value(HandlePtr) of registerCallback. Otherwise the callback is unregistered when the return value is deleted.