Introduce node names, allowing a node to have multiple IP addresses.
Also, make the front-end code IPv6-aware and support AAAA records.

This is an attempt to fix a problem where sources would become stuck
on master -> slave state transitions.

This allows running the service on a single port: with the
--enable-icecast-proxy option, all requests can go through
redirectord.

Make progress anyway when the history is gone.

go-etcd now closes the channel passed to Watch() on error, so we need
to restart the syncer every time and use a temp channel.