Trigger config-changed on address change

Trigger a `config-changed` event whenever a unit's addresses change. This is a step on the road to solving https://launchpad.net/bugs/1215579 (propagating address changes to relation settings).

There's a new uniter API method here, `WatchUnitAddresses`, which on the server-side simply defers to `state.Machine.WatchAddresses`. The uniter filter is updated slightly so that it starts watching address changes after the first `config-changed` event is received.
> Am I making sense here?

Not sure if I follow. If we handle all address changes from the beginning, then we're guaranteed to get a double config-changed (one from the initial address-changed, one from the initial config-changed).

I did it this way to preserve the behaviour of not firing `config-changed` until the charm URL is set. The initial address-changed event is ignored because it'll be handled by the config-changed that is about to happen.