subnets: move forward the cursor to skip illegal subnet

This PR fixs an issue when flannel gets illegal subnet event in
watching leases, it doesn't move forward the etcd cursor and
will stuck in the same invalid event forever.

subnet/etcdv2/local_manager.go

@@ -345,6 +345,9 @@ func (m *LocalManager) WatchLeases(ctx context.Context, cursor interface{}) (Lea
 		log.Warning("Watch of subnet leases failed because etcd index outside history window")
 		return m.leasesWatchReset(ctx)
 
+	case index != 0:
+		return LeaseWatchResult{Cursor: watchCursor{index}}, err
+
 	default:
 		return LeaseWatchResult{}, err
 	}

subnet/watch.go

@@ -40,6 +40,10 @@ func WatchLeases(ctx context.Context, sm Manager, ownLease *Lease, receiver chan
 				return
 			}
 
+			if res.Cursor != nil {
+				cursor = res.Cursor
+			}
+
 			log.Errorf("Watch subnets: %v", err)
 			time.Sleep(time.Second)
 			continue