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.

htlcswitch/switch_test.go

@@ -1417,7 +1417,7 @@ func testSkipLinkLocalForward(t *testing.T, eligible bool,
 	// We'll attempt to send out a new HTLC that has Alice as the first
 	// outgoing link. This should fail as Alice isn't yet able to forward
 	// any active HTLC's.
-	err = s.SendHTLC(aliceChannelLink.ShortChanID(), 0, addMsg, nil)
+	err = s.SendHTLC(aliceChannelLink.ShortChanID(), 0, addMsg)
 	if err == nil {
 		t.Fatalf("local forward should fail due to inactive link")
 	}
@@ -1742,7 +1742,9 @@ func TestSwitchSendPayment(t *testing.T) {
 
 	// First check that the switch will correctly respond that this payment
 	// ID is unknown.
-	_, err = s.GetPaymentResult(paymentID)
+	_, err = s.GetPaymentResult(
+		paymentID, newMockDeobfuscator(),
+	)
 	if err != ErrPaymentIDNotFound {
 		t.Fatalf("expected ErrPaymentIDNotFound, got %v", err)
 	}
@@ -1752,19 +1754,25 @@ func TestSwitchSendPayment(t *testing.T) {
 	go func() {
 		err := s.SendHTLC(
 			aliceChannelLink.ShortChanID(), paymentID, update,
-			newMockDeobfuscator())
+		)
 		if err != nil {
 			errChan <- err
 			return
 		}
 
-		resultChan, err := s.GetPaymentResult(paymentID)
+		resultChan, err := s.GetPaymentResult(
+			paymentID, newMockDeobfuscator(),
+		)
 		if err != nil {
 			errChan <- err
 			return
 		}
 
-		result := <-resultChan
+		result, ok := <-resultChan
+		if !ok {
+			errChan <- fmt.Errorf("shutting down")
+		}
+
 		if result.Error != nil {
 			errChan <- result.Error
 			return