So we are always returning a set of unique Endpoints.

Use pb.Status as the node information.

At least it doesn't require sudo like using systemctl.

And remove the unused StringSet type.

Export the node Status as metrics.

Also add instrumentation for the load balancer module.

We only use the proxy for Icecast streaming so there's no need to be
generic.

Includes a merge of radiod and redirectord, updates to more modern Go
idioms, adoption of the higher level etcd concurrency primitives, and
a lot of other minor implementation changes.

The refactor is still incomplete: the daemons won't compile and the
code to start the RPC servers is missing.

Add an option to load a file containing redirects (source, target paths)
and serve them from the HTTP server, to allow renaming a radio without
breaking old links.