switch+router+server: move NextPaymentID to router

This commit moves the responsibility of generating a unique payment ID
from the switch to the router. This will make it easier for the router
to keep track of which HTLCs were successfully forwarded onto the
network, as it can query the switch for existing HTLCs as long as the
paymentIDs are kept.

The router is expected to maintain a map from paymentHash->paymentID,
such that they can be replayed on restart. This also lets the router
check the status of a sent payment after a restart, by querying the
switch for the paymentID in question.

htlcswitch/test_utils.go

@@ -543,7 +543,7 @@ func getChanID(msg lnwire.Message) (lnwire.ChannelID, error) {
 func generatePaymentWithPreimage(invoiceAmt, htlcAmt lnwire.MilliSatoshi,
 	timelock uint32, blob [lnwire.OnionPacketSize]byte,
 	preimage, rhash [32]byte) (*channeldb.Invoice, *lnwire.UpdateAddHTLC,
-	error) {
+	uint64, error) {
 
 	// Create the db invoice. Normally the payment requests needs to be set,
 	// because it is decoded in InvoiceRegistry to obtain the cltv expiry.
@@ -566,18 +566,25 @@ func generatePaymentWithPreimage(invoiceAmt, htlcAmt lnwire.MilliSatoshi,
 		OnionBlob:   blob,
 	}
 
-	return invoice, htlc, nil
+	pid, err := generateRandomBytes(8)
+	if err != nil {
+		return nil, nil, 0, err
+	}
+	paymentID := binary.BigEndian.Uint64(pid)
+
+	return invoice, htlc, paymentID, nil
 }
 
 // generatePayment generates the htlc add request by given path blob and
 // invoice which should be added by destination peer.
 func generatePayment(invoiceAmt, htlcAmt lnwire.MilliSatoshi, timelock uint32,
-	blob [lnwire.OnionPacketSize]byte) (*channeldb.Invoice, *lnwire.UpdateAddHTLC, error) {
+	blob [lnwire.OnionPacketSize]byte) (*channeldb.Invoice,
+	*lnwire.UpdateAddHTLC, uint64, error) {
 
 	var preimage [sha256.Size]byte
 	r, err := generateRandomBytes(sha256.Size)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, 0, err
 	}
 	copy(preimage[:], r)
 
@@ -772,7 +779,9 @@ func preparePayment(sendingPeer, receivingPeer lnpeer.Peer,
 	}
 
 	// Generate payment: invoice and htlc.
-	invoice, htlc, err := generatePayment(invoiceAmt, htlcAmt, timelock, blob)
+	invoice, htlc, pid, err := generatePayment(
+		invoiceAmt, htlcAmt, timelock, blob,
+	)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -786,7 +795,7 @@ func preparePayment(sendingPeer, receivingPeer lnpeer.Peer,
 	// Send payment and expose err channel.
 	return invoice, func() error {
 		_, err := sender.htlcSwitch.SendHTLC(
-			firstHop, htlc, newMockDeobfuscator(),
+			firstHop, pid, htlc, newMockDeobfuscator(),
 		)
 		return err
 	}, nil
@@ -1235,8 +1244,10 @@ func (n *twoHopNetwork) makeHoldPayment(sendingPeer, receivingPeer lnpeer.Peer,
 	rhash := preimage.Hash()
 
 	// Generate payment: invoice and htlc.
-	invoice, htlc, err := generatePaymentWithPreimage(invoiceAmt, htlcAmt, timelock, blob,
-		channeldb.UnknownPreimage, rhash)
+	invoice, htlc, pid, err := generatePaymentWithPreimage(
+		invoiceAmt, htlcAmt, timelock, blob,
+		channeldb.UnknownPreimage, rhash,
+	)
 	if err != nil {
 		paymentErr <- err
 		return paymentErr
@@ -1251,7 +1262,7 @@ func (n *twoHopNetwork) makeHoldPayment(sendingPeer, receivingPeer lnpeer.Peer,
 	// Send payment and expose err channel.
 	go func() {
 		_, err := sender.htlcSwitch.SendHTLC(
-			firstHop, htlc, newMockDeobfuscator(),
+			firstHop, pid, htlc, newMockDeobfuscator(),
 		)
 		paymentErr <- err
 	}()