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

htlcswitch/link_test.go

@@ -14,10 +14,10 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"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"
@@ -70,25 +70,6 @@ func (c *concurrentTester) Fatalf(format string, args ...interface{}) {
 // setting the 'Curve" parameter to nil. Doing this avoids printing out each of
 // the field elements in the curve parameters for secp256k1.
 func messageToString(msg lnwire.Message) string {
-	switch m := msg.(type) {
-	case *lnwire.RevokeAndAck:
-		m.NextRevocationKey.Curve = nil
-	case *lnwire.AcceptChannel:
-		m.FundingKey.Curve = nil
-		m.RevocationPoint.Curve = nil
-		m.PaymentPoint.Curve = nil
-		m.DelayedPaymentPoint.Curve = nil
-		m.FirstCommitmentPoint.Curve = nil
-	case *lnwire.OpenChannel:
-		m.FundingKey.Curve = nil
-		m.RevocationPoint.Curve = nil
-		m.PaymentPoint.Curve = nil
-		m.DelayedPaymentPoint.Curve = nil
-		m.FirstCommitmentPoint.Curve = nil
-	case *lnwire.FundingLocked:
-		m.NextPerCommitmentPoint.Curve = nil
-	}
-
 	return spew.Sdump(msg)
 }