rpcserver+routerrpc: add subscribe events to router backend

Add SubscribeHtlcEvents to RouterBackend in preparation for the
addition of a HtlcNotifier stream.

rpcserver.go

@@ -567,6 +567,7 @@ func newRPCServer(s *server, macService *macaroons.Service,
 		Tower:                 s.controlTower,
 		MaxTotalTimelock:      cfg.MaxOutgoingCltvExpiry,
 		DefaultFinalCltvDelta: uint16(cfg.Bitcoin.TimeLockDelta),
+		SubscribeHtlcEvents:   s.htlcNotifier.SubscribeHtlcEvents,
 	}
 
 	genInvoiceFeatures := func() *lnwire.FeatureVector {