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/test_utils.go

@@ -795,17 +795,23 @@ func preparePayment(sendingPeer, receivingPeer lnpeer.Peer,
 	// Send payment and expose err channel.
 	return invoice, func() error {
 		err := sender.htlcSwitch.SendHTLC(
-			firstHop, pid, htlc, newMockDeobfuscator(),
+			firstHop, pid, htlc,
 		)
 		if err != nil {
 			return err
 		}
-		resultChan, err := sender.htlcSwitch.GetPaymentResult(pid)
+		resultChan, err := sender.htlcSwitch.GetPaymentResult(
+			pid, newMockDeobfuscator(),
+		)
 		if err != nil {
 			return err
 		}
 
-		result := <-resultChan
+		result, ok := <-resultChan
+		if !ok {
+			return fmt.Errorf("shutting down")
+		}
+
 		if result.Error != nil {
 			return result.Error
 		}
@@ -1275,20 +1281,26 @@ func (n *twoHopNetwork) makeHoldPayment(sendingPeer, receivingPeer lnpeer.Peer,
 	// Send payment and expose err channel.
 	go func() {
 		err := sender.htlcSwitch.SendHTLC(
-			firstHop, pid, htlc, newMockDeobfuscator(),
+			firstHop, pid, htlc,
 		)
 		if err != nil {
 			paymentErr <- err
 			return
 		}
 
-		resultChan, err := sender.htlcSwitch.GetPaymentResult(pid)
+		resultChan, err := sender.htlcSwitch.GetPaymentResult(
+			pid, newMockDeobfuscator(),
+		)
 		if err != nil {
 			paymentErr <- err
 			return
 		}
 
-		result := <-resultChan
+		result, ok := <-resultChan
+		if !ok {
+			paymentErr <- fmt.Errorf("shutting down")
+		}
+
 		if result.Error != nil {
 			paymentErr <- result.Error
 			return