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

chanfitness/chaneventstore_test.go

@@ -2,11 +2,10 @@ package chanfitness
 
 import (
 	"errors"
-	"math/big"
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/clock"
@@ -88,11 +87,10 @@ func TestStartStoreError(t *testing.T) {
 // of uptime and lifespan requests, as they are tested in their own tests.
 func TestMonitorChannelEvents(t *testing.T) {
 	var (
-		pubKey = &btcec.PublicKey{
-			X:     big.NewInt(0),
-			Y:     big.NewInt(1),
-			Curve: btcec.S256(),
-		}
+		pubKey = btcec.NewPublicKey(
+			new(btcec.FieldVal).SetInt(0),
+			new(btcec.FieldVal).SetInt(1),
+		)
 
 		chan1 = wire.OutPoint{Index: 1}
 		chan2 = wire.OutPoint{Index: 2}

chanfitness/chaneventstore_testctx_test.go

@@ -1,11 +1,10 @@
 package chanfitness
 
 import (
-	"math/big"
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/channeldb"
@@ -144,11 +143,10 @@ func (c *chanEventStoreTestCtx) newChannel() (route.Vertex, *btcec.PublicKey,
 	wire.OutPoint) {
 
 	// Create a pubkey for our channel peer.
-	pubKey := &btcec.PublicKey{
-		X:     big.NewInt(int64(c.testVarIdx)),
-		Y:     big.NewInt(int64(c.testVarIdx)),
-		Curve: btcec.S256(),
-	}
+	pubKey := btcec.NewPublicKey(
+		new(btcec.FieldVal).SetInt(uint16(c.testVarIdx)),
+		new(btcec.FieldVal).SetInt(uint16(c.testVarIdx)),
+	)
 
 	// Create vertex from our pubkey.
 	vertex, err := route.NewVertexFromBytes(pubKey.SerializeCompressed())