Master election code should be more robust now, this commit eliminates
the discrepancy between role and master info (previously it was possible
for these two to be non synchronized).

This commit also includes an in-memory etcd mock, which implements
enough of the etcd interface to test the masterelection code.

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.