WiP: Add uniter changes to support reboot

This PR adds the uniter changes needed to support reboot.

Depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
https://github.com/juju/juju/pull/782
https://github.com/juju/juju/pull/833

please don't make `writeChanges` a lie. Rather than making it depend on `writeChanges` *and something else*, fold that something else into writeChanges when you set it up.