rpcserver: let sendPayment sendLoop listen for shutdown

Intead of checking for shutdown in the receive loop, we let the sendLoop
handle it, as it can return the error directly. This works since the
returning sendLoop will trigger a close of the `reqQuit` channel, which
will ensure the receive loop exits.

rpcserver.go

@@ -3696,9 +3696,7 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 			select {
 			case <-reqQuit:
 				return
-			case <-r.quit:
-				errChan <- nil
-				return
+
 			default:
 				// Receive the next pending payment within the
 				// stream sent by the client. If we read the
@@ -3756,6 +3754,9 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 		case err := <-errChan:
 			return err
 
+		case <-r.quit:
+			return errors.New("rpc server shutting down")
+
 		case payIntent := <-payChan:
 			// We launch a new goroutine to execute the current
 			// payment so we can continue to serve requests while