Destroy event history collectors

The EventHistoryCollector objects are tied to a session and there is a max per-session.
Without destroying the collectors, we could max out the collectors causing 'govc events' to return empty.

Fixes #948
Fixes #961

event/manager.go

@@ -180,5 +180,7 @@ func (m Manager) Events(ctx context.Context, objects []types.ManagedObjectRefere
 		proc.addObject(ctx, o)
 	}
 
+	defer proc.destroy()
+
 	return proc.run(ctx, tail)
 }

event/processor.go

@@ -28,7 +28,7 @@ import (
 type tailInfo struct {
 	t         *eventTailer
 	obj       types.ManagedObjectReference
-	collector types.ManagedObjectReference
+	collector *HistoryCollector
 }
 
 type eventProcessor struct {
@@ -68,20 +68,26 @@ func (p *eventProcessor) addObject(ctx context.Context, obj types.ManagedObjectR
 	p.tailers[collector.Reference()] = &tailInfo{
 		t:         newEventTailer(),
 		obj:       obj,
-		collector: collector.Reference(),
+		collector: collector,
 	}
 
 	return nil
 }
 
+func (p *eventProcessor) destroy() {
+	for _, info := range p.tailers {
+		_ = info.collector.Destroy(context.Background())
+	}
+}
+
 func (p *eventProcessor) run(ctx context.Context, tail bool) error {
 	if len(p.tailers) == 0 {
 		return nil
 	}
 
 	var collectors []types.ManagedObjectReference
 	for _, t := range p.tailers {
-		collectors = append(collectors, t.collector)
+		collectors = append(collectors, t.collector.Reference())
 	}
 
 	c := property.DefaultCollector(p.mgr.Client())