routing: add methods checkTimeout and requestRoute

This commit refactors the `resumePayment` method by adding the methods
`checkTimeout` and `requestRoute` so it's easier to understand the flow
and reason about the error handling.

routing/mock_test.go

@@ -673,6 +673,12 @@ func (m *mockPaymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 	activeShards, height uint32) (*route.Route, error) {
 
 	args := m.Called(maxAmt, feeLimit, activeShards, height)
+
+	// Type assertion on nil will fail, so we check and return here.
+	if args.Get(0) == nil {
+		return nil, args.Error(1)
+	}
+
 	return args.Get(0).(*route.Route), args.Error(1)
 }