engine: trigger engine reconciliation on job target state changes

@bcwaldon thoughts on something like this? Small step towards making polvi happier...
Ah, when I first did this patch that function had been removed.

Sort of - it's just a bit messy since the filter "interface" only returns a single event, so then we need
```
func (e *Engine) HandleCommandStopJob(ev event.Event)
func (e *Engine) HandleCommandStartJob(ev event.Event)
func (e *Engine) HandleJobTargetStateChange(ev event.Event)
```

... or am I missing something?