proxy: improved service discovery

[Brian Martin]

The proxy implemented in #416 needs to handle service discovery. The zookeeper service discovery included in the initial implementation is very Aurora/Finagle specific.

One option to improve this is to expand on the admin commands and send topology changes from a sidecar process to the proxy via the admin port. This would allow for custom handling of service discovery without the need to support a wide variety of approaches within the proxy.

As part of this, we need to also be able to handle changes to the serverset and ability to provide some key to hash on (eg shard id)