service relation lifecycle api

[William Reade]

The first client that desperately needs lifecycles to work is the unit agent, which needs to track relation lifecycle. I hope I'm not going to regret this, but I *think* that all I need is a ServiceRelationsWatcher that watches a specific service's relations throughout their lifecycle and produces these:

type RelationsChange struct {
    Changed []*Relation
}

The initial event should contain every relation, whatever its state; subsequent ones should contain only the relations whose state has changed since the last event. I have come to the conclusion that I actually don't need separate Alive/Dying/Dead fields in the event.

AIUI, it will need to:

* watch the topology for changes to the service's relations as it currently does
* on relevant topology change:
  * for each added relation:
    * store its life status, and start a life watch *unless* it is already dead
  * report all added relations in a single event
* on life change (that was started on new topology relation)
  * if life status is Dead, stop the relation's life watch
  * report the single changed relation in a single event

In my mind, this kinda assumes a LifeWatcher type that guarantees it won't send the same status more than once, but guarantees it *will* send a Dead event if the node is deleted before it notices a change to Dead.

(Relation needs Life and SetLife, or whatever it was we agreed on.)

[Frank Mueller (themue)]

ServiceRelationsWatcher exists but is not yet used.