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.
I can't see when this would be set to nil -- do we need to check it?