multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

routing/router.go

@@ -10,9 +10,9 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/go-errors/errors"
 	sphinx "github.com/lightningnetwork/lightning-onion"
@@ -1803,7 +1803,7 @@ func generateNewSessionKey() (*btcec.PrivateKey, error) {
 	// any replay.
 	//
 	// TODO(roasbeef): add more sources of randomness?
-	return btcec.NewPrivateKey(btcec.S256())
+	return btcec.NewPrivateKey()
 }
 
 // generateSphinxPacket generates then encodes a sphinx packet which encodes
@@ -1827,7 +1827,6 @@ func generateSphinxPacket(rt *route.Route, paymentHash []byte,
 			path := make([]sphinx.OnionHop, sphinxPath.TrueRouteLength())
 			for i := range path {
 				hopCopy := sphinxPath[i]
-				hopCopy.NodePub.Curve = nil
 				path[i] = hopCopy
 			}
 			return spew.Sdump(path)
@@ -1857,7 +1856,6 @@ func generateSphinxPacket(rt *route.Route, paymentHash []byte,
 			// internal curve here in order to keep the logs from
 			// getting noisy.
 			key := *sphinxPacket.EphemeralKey
-			key.Curve = nil
 			packetCopy := *sphinxPacket
 			packetCopy.EphemeralKey = &key
 			return spew.Sdump(packetCopy)
@@ -2071,7 +2069,6 @@ func spewPayment(payment *LightningPayment) logClosure {
 			var hopHints []zpay32.HopHint
 			for _, hopHint := range routeHint {
 				h := hopHint.Copy()
-				h.NodeID.Curve = nil
 				hopHints = append(hopHints, h)
 			}
 			routeHints = append(routeHints, hopHints)