Agent presence timeouts should fire relation hooks

[Dave Cheney]

The current Juju model does not cope with machines being removed manually behind the scenes.

In our EC2/HP Cloud world this is rarely a problem, but is more common in the MaaS world where people are predisposed to percussive maintenance. See, LP #1206532

Another more subtle effect of this is relation-departed/broken and peer relation hooks do not fire to signal that a backend service unit has gone away.

Obviously, the charm should make an attempt at coping with this; but if we made the charm authors do everything, there would be nothing left for us to do :) And being more realistic, charm authors can only work within the constraints of the software they are charming, memcache for instance has not heartbeat mechanism.

One solution is to hook the agent presence notification system in to the relation system, this would not solve the phantom machine problem from LP # 1206532, but would allow some level of automagic healing for Juju environments.