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.

routing/router.go

@@ -134,6 +134,7 @@ type PaymentAttemptDispatcher interface {
 	// denoted by its public key. A non-nil error is to be returned if the
 	// payment was unsuccessful.
 	SendHTLC(firstHop lnwire.ShortChannelID,
+		paymentID uint64,
 		htlcAdd *lnwire.UpdateAddHTLC,
 		deobfuscator htlcswitch.ErrorDecrypter) ([sha256.Size]byte, error)
 }
@@ -208,6 +209,12 @@ type Config struct {
 	// returned.
 	QueryBandwidth func(edge *channeldb.ChannelEdgeInfo) lnwire.MilliSatoshi
 
+	// NextPaymentID is a method that guarantees to return a new, unique ID
+	// each time it is called. This is used by the router to generate a
+	// unique payment ID for each payment it attempts to send, such that
+	// the switch can properly handle the HTLC.
+	NextPaymentID func() (uint64, error)
+
 	// AssumeChannelValid toggles whether or not the router will check for
 	// spentness of channel outpoints. For neutrino, this saves long rescans
 	// from blocking initial usage of the daemon.
@@ -1715,8 +1722,15 @@ func (r *ChannelRouter) sendToSwitch(route *route.Route, paymentHash [32]byte) (
 		OnionErrorDecrypter: sphinx.NewOnionErrorDecrypter(circuit),
 	}
 
+	// We generate a new, unique payment ID that we will use for
+	// this HTLC.
+	paymentID, err := r.cfg.NextPaymentID()
+	if err != nil {
+		return [32]byte{}, err
+	}
+
 	return r.cfg.Payer.SendHTLC(
-		firstHop, htlcAdd, errorDecryptor,
+		firstHop, paymentID, htlcAdd, errorDecryptor,
 	)
 }