[#378] shard: Define new epoch event

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/shard/gc.go

@@ -21,8 +21,24 @@ type eventType int
 
 const (
 	_ eventType = iota
+	eventNewEpoch
 )
 
+type newEpoch struct {
+	epoch uint64
+}
+
+func (e newEpoch) typ() eventType {
+	return eventNewEpoch
+}
+
+// EventNewEpoch returns new epoch event.
+func EventNewEpoch(e uint64) Event {
+	return newEpoch{
+		epoch: e,
+	}
+}
+
 type eventHandler func(context.Context, Event)
 
 type eventHandlers struct {