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
Merge pull request #343 from bcwaldon/ConditionMachineOf

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