engine blindly reoffers jobs when unscheduled

[bcwaldon]

When an agent punts its jobs due to shutdown, inability to run it, etc, the engine reacts to the event by re-offering it [0]. This is generally correct, but fleet is not doing this operation safely at all. All engines end up re-offering the job, causing a lot of offer/bid churn and lots of misleading logging.

[0]

fleet/engine/event.go

Line 53 in e0506c9

func (eh *EventHandler) HandleEventJobUnscheduled(ev event.Event) {