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

peer/test_utils.go

@@ -12,10 +12,10 @@ 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/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/htlcswitch"
@@ -64,13 +64,13 @@ func createTestPeer(notifier chainntnfs.ChainNotifier,
 		Family: keychain.KeyFamilyNodeKey,
 	}
 	aliceKeyPriv, aliceKeyPub := btcec.PrivKeyFromBytes(
-		btcec.S256(), channels.AlicesPrivKey,
+		channels.AlicesPrivKey,
 	)
 	aliceKeySigner := keychain.NewPrivKeyMessageSigner(
 		aliceKeyPriv, nodeKeyLocator,
 	)
 	bobKeyPriv, bobKeyPub := btcec.PrivKeyFromBytes(
-		btcec.S256(), channels.BobsPrivKey,
+		channels.BobsPrivKey,
 	)
 
 	channelCapacity := btcutil.Amount(10 * 1e8)