routing/payment_session: make RequestRoute take max amt, fee limit and

active shards

In preparation for doing pathfinding for routes sending a value less
than the total payment amount, we let the payment session take the max
amount to send and the fee limit as arguments to RequestRoute.

routing/mock_test.go

@@ -122,7 +122,9 @@ type mockPaymentSession struct {
 
 var _ PaymentSession = (*mockPaymentSession)(nil)
 
-func (m *mockPaymentSession) RequestRoute(height uint32) (*route.Route, error) {
+func (m *mockPaymentSession) RequestRoute(_, _ lnwire.MilliSatoshi,
+	_, height uint32) (*route.Route, error) {
+
 	if len(m.routes) == 0 {
 		return nil, fmt.Errorf("no routes")
 	}