[hackernews example] Pull round-robin logic from Docker compose to HAproxy

[bennostein]

This PR hands control of load-balancing back to ... the load-balancer, routing web-service -> skip traffic through the reverse proxy. That also means the web service now need not care whether the skip service is running in "distributed mode" or not.

@beauby I think this is more or less what you had in mind -- what do you think? One thing I didn't bother with yet that is probably important in practice is locking down the :8081 listeners in the haproxy configs to not accept any external IPs.