discovery+server: RetransmitDelay->RetransmitTicker

Also let retransmitStaleChannels take a timestamp, to make it easier to
test.

server.go

@@ -719,7 +719,7 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB,
 		NotifyWhenOffline:       s.NotifyWhenOffline,
 		ProofMatureDelta:        0,
 		TrickleDelay:            time.Millisecond * time.Duration(cfg.TrickleDelay),
-		RetransmitDelay:         time.Minute * 30,
+		RetransmitTicker:        ticker.New(time.Minute * 30),
 		RebroadcastInterval:     time.Hour * 24,
 		WaitingProofStore:       waitingProofStore,
 		MessageStore:            gossipMessageStore,