routerrpc: convert sendpayment to async

Modify the routerrpc SendPayment api to asynchronous. This allows
callers to pick up a payment after the rpc connection was lost or lnd
was restarted.

server.go

@@ -192,6 +192,8 @@ type server struct {
 
 	chanRouter *routing.ChannelRouter
 
+	controlTower routing.ControlTower
+
 	authGossiper *discovery.AuthenticatedGossiper
 
 	utxoNursery *utxoNursery
@@ -651,12 +653,14 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB, cc *chainControl,
 
 	paymentControl := channeldb.NewPaymentControl(chanDB)
 
+	s.controlTower = routing.NewControlTower(paymentControl)
+
 	s.chanRouter, err = routing.New(routing.Config{
 		Graph:              chanGraph,
 		Chain:              cc.chainIO,
 		ChainView:          cc.chainView,
 		Payer:              s.htlcSwitch,
-		Control:            routing.NewControlTower(paymentControl),
+		Control:            s.controlTower,
 		MissionControl:     s.missionControl,
 		ChannelPruneExpiry: routing.DefaultChannelPruneExpiry,
 		GraphPruneInterval: time.Duration(time.Hour),