Provide a proxy for generic service discovery

While Announcer provides service registration, we lack a cross-cutting answer for service discovery. There are well-known libraries that will do it (e.g. finagle), but we need an answer for others. Marathon, for example, provides a script called haproxy_marathon_bridge that reloads configuration of HAProxy for this purpose. We could do something similar with a mixin Process that dynamically routes an inbound port to a serverset path in ZooKeeper.