fix(scheduling): Correct the ConditionMachineOf behavior (mostly)

* Stop creating job offers in the agents, just clear a job's target
* Add EventJobUnscheduled, facilitating communication of need to reschedule
* Only clear a job's target if it the target is known
feat(payload): support timer units

* Allow users to submit systemd timer units
* Add a default peer to systemd timer units similar to the
  handling of socket units
Merge pull request #305 from bcwaldon/timer-units

Support timer units
Merge pull request #343 from bcwaldon/ConditionMachineOf

fix(scheduling): Correct the ConditionMachineOf behavior (mostly)
fix(fleetctl): Make fleetctl stop/unload more helpful

Like load and start, unload and stop should block on success.
Additionally, unload should attempt to stop a job before
fully unloading it.