private void removeOldClients() {
        synchronized (monitor) {
            for (var iterator = clientFeederByClientId.values().iterator(); iterator.hasNext(); ) {
                ClientFeederV3 client = iterator.next();
                if (client.timedOut()) {
                    client.kill();
                    iterator.remove();
                }
            }
        }
    }
One client has one ClientFeederV3 shared between all client threads.
Contains logic for shutting down cleanly as the server is upgraded.

@author Haakon Dybdahl
vespaclient-container-plugin/src/main/java/com/yahoo/vespa/http/server/FeedHandlerV3.java