refactor RadioNode for robustness

Increase robustness of the config watcher by reloading the full
configuration on Etcd errors (fixes an issue with the very common error
401).

Make sure that the Icecast config reload is only triggered if the
configuration has actually changed.

Added a way to stop the node properly, which ensures that we are
tracking all the resources and that there are no deadlocks in the
internal communications.

MasterElection now closes the update channel when it's done.