rpcserver+routerrpc: add subscribe events to router backend

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

lnrpc/routerrpc/router_backend.go

@@ -20,6 +20,7 @@ import (
 	"github.com/lightningnetwork/lnd/record"
 	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/routing/route"
+	"github.com/lightningnetwork/lnd/subscribe"
 	"github.com/lightningnetwork/lnd/zpay32"
 )
 
@@ -68,6 +69,10 @@ type RouterBackend struct {
 	// DefaultFinalCltvDelta is the default value used as final cltv delta
 	// when an RPC caller doesn't specify a value.
 	DefaultFinalCltvDelta uint16
+
+	// SubscribeHtlcEvents returns a subscription client for the node's
+	// htlc events.
+	SubscribeHtlcEvents func() (*subscribe.Client, error)
 }
 
 // MissionControl defines the mission control dependencies of routerrpc.

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 {