p2p/discv5: add delay to refresh cycle when no seed nodes are found (#16994)

p2p/discv5/net.go

@@ -678,7 +678,7 @@ func (net *Network) refresh(done chan<- struct{}) {
 	}
 	if len(seeds) == 0 {
 		log.Trace("no seed nodes found")
-		close(done)
+		time.AfterFunc(time.Second*10, func() { close(done) })
 		return
 	}
 	for _, n := range seeds {