htlcswitch+router: move deobfuscator creation to GetPaymentResult call

In this commit we move handing the deobfuscator from the router to the
switch from when the payment is initiated, to when the result is
queried.

We do this because only the router can recreate the deobfuscator after a
restart, and we are preparing for being able to handle results across
restarts.

Since the deobfuscator cannot be nil anymore, we can also get rid of
that special case.

routing/mock_test.go

@@ -14,8 +14,7 @@ var _ PaymentAttemptDispatcher = (*mockPaymentAttemptDispatcher)(nil)
 
 func (m *mockPaymentAttemptDispatcher) SendHTLC(firstHop lnwire.ShortChannelID,
 	pid uint64,
-	_ *lnwire.UpdateAddHTLC,
-	_ htlcswitch.ErrorDecrypter) error {
+	_ *lnwire.UpdateAddHTLC) error {
 
 	if m.onPayment == nil {
 		return nil
@@ -44,8 +43,8 @@ func (m *mockPaymentAttemptDispatcher) SendHTLC(firstHop lnwire.ShortChannelID,
 	return nil
 }
 
-func (m *mockPaymentAttemptDispatcher) GetPaymentResult(paymentID uint64) (
-	<-chan *htlcswitch.PaymentResult, error) {
+func (m *mockPaymentAttemptDispatcher) GetPaymentResult(paymentID uint64,
+	_ htlcswitch.ErrorDecrypter) (<-chan *htlcswitch.PaymentResult, error) {
 
 	c := make(chan *htlcswitch.PaymentResult, 1)
 	res, ok := m.results[paymentID]