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/link_test.go

@@ -1112,21 +1112,26 @@ func TestChannelLinkMultiHopUnknownPaymentHash(t *testing.T) {
 	// Send payment and expose err channel.
 	err = n.aliceServer.htlcSwitch.SendHTLC(
 		n.firstBobChannelLink.ShortChanID(), pid, htlc,
-		newMockDeobfuscator(),
 	)
 	if err != nil {
 		t.Fatalf("unable to get send payment: %v", err)
 	}
 
-	resultChan, err := n.aliceServer.htlcSwitch.GetPaymentResult(pid)
+	resultChan, err := n.aliceServer.htlcSwitch.GetPaymentResult(
+		pid, newMockDeobfuscator(),
+	)
 	if err != nil {
 		t.Fatalf("unable to get payment result: %v", err)
 	}
 
 	var result *PaymentResult
+	var ok bool
 	select {
 
-	case result = <-resultChan:
+	case result, ok = <-resultChan:
+		if !ok {
+			t.Fatalf("unexpected shutdown")
+		}
 	case <-time.After(5 * time.Second):
 		t.Fatalf("no result arrive")
 	}
@@ -3888,19 +3893,24 @@ func TestChannelLinkAcceptDuplicatePayment(t *testing.T) {
 	// properly.
 	err = n.aliceServer.htlcSwitch.SendHTLC(
 		n.firstBobChannelLink.ShortChanID(), pid, htlc,
-		newMockDeobfuscator(),
 	)
 	if err != nil {
 		t.Fatalf("unable to send payment to carol: %v", err)
 	}
 
-	resultChan, err := n.aliceServer.htlcSwitch.GetPaymentResult(pid)
+	resultChan, err := n.aliceServer.htlcSwitch.GetPaymentResult(
+		pid, newMockDeobfuscator(),
+	)
 	if err != nil {
 		t.Fatalf("unable to get payment result: %v", err)
 	}
 
 	select {
-	case result := <-resultChan:
+	case result, ok := <-resultChan:
+		if !ok {
+			t.Fatalf("unexpected shutdown")
+		}
+
 		if result.Error != nil {
 			t.Fatalf("payment failed: %v", result.Error)
 		}
@@ -3912,7 +3922,6 @@ func TestChannelLinkAcceptDuplicatePayment(t *testing.T) {
 	// as it's a duplicate request.
 	err = n.aliceServer.htlcSwitch.SendHTLC(
 		n.firstBobChannelLink.ShortChanID(), pid, htlc,
-		newMockDeobfuscator(),
 	)
 	if err != ErrAlreadyPaid {
 		t.Fatalf("ErrAlreadyPaid should have been received got: %v", err)