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.

rpcserver.go

@@ -477,6 +477,7 @@ func newRPCServer(s *server, macService *macaroons.Service,
 		FindRoute:       s.chanRouter.FindRoute,
 		MissionControl:  s.missionControl,
 		ActiveNetParams: activeNetParams.Params,
+		Tower:           s.controlTower,
 	}
 
 	var (