@Override
        public void onChange(List<DiscoveryResult> removedSet) {
            if (!removedSet.isEmpty()) {
                LOG.debug(
                        "Removing connection pools for missing servers. name = {}. {} servers gone.",
                        originName,
                        removedSet.size());
                for (DiscoveryResult s : removedSet) {
                    IConnectionPool pool = perServerPools.remove(s);
                    if (pool != null) {
                        pool.shutdown();
                    }
                }
            }
        }
Gracefully shuts down a DefaultClientChannelManager by allowing in-flight requests to finish before closing the connections.
Idle connections in the connection pools are closed, and any connections associated with an in-flight request
will be closed upon trying to return the connection to the pool
zuul-core/src/main/java/com/netflix/zuul/netty/connectionpool/DefaultClientChannelManager.java