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

lnwallet/channel.go

@@ -11,13 +11,14 @@ import (
 	"sync"
 
 	"github.com/btcsuite/btcd/blockchain"
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/txsort"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btclog"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/txsort"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/chainntnfs"
@@ -303,7 +304,7 @@ type PaymentDescriptor struct {
 	// local node. This signature is generated by the remote node and
 	// stored by the local node in the case that local node needs to
 	// broadcast their commitment transaction.
-	sig *btcec.Signature
+	sig *ecdsa.Signature
 
 	// addCommitHeight[Remote|Local] encodes the height of the commitment
 	// which included this HTLC on either the remote or local commitment
@@ -2297,7 +2298,7 @@ func NewBreachRetribution(chanState *channeldb.OpenChannel, stateNum uint64,
 		return nil, err
 	}
 	commitmentSecret, commitmentPoint := btcec.PrivKeyFromBytes(
-		btcec.S256(), revocationPreimage[:],
+		revocationPreimage[:],
 	)
 
 	// With the commitment point generated, we can now generate the four
@@ -4105,7 +4106,7 @@ func genHtlcSigValidationJobs(localCommitmentView *commitment,
 		var (
 			htlcIndex uint64
 			sigHash   func() ([]byte, error)
-			sig       *btcec.Signature
+			sig       *ecdsa.Signature
 			err       error
 		)
 
@@ -4421,16 +4422,13 @@ func (lc *LightningChannel) ReceiveNewCommitment(commitSig lnwire.Sig,
 	// While the HTLC verification jobs are proceeding asynchronously,
 	// we'll ensure that the newly constructed commitment state has a valid
 	// signature.
-	verifyKey := btcec.PublicKey{
-		X:     lc.channelState.RemoteChanCfg.MultiSigKey.PubKey.X,
-		Y:     lc.channelState.RemoteChanCfg.MultiSigKey.PubKey.Y,
-		Curve: btcec.S256(),
-	}
+	verifyKey := lc.channelState.RemoteChanCfg.MultiSigKey.PubKey
+
 	cSig, err := commitSig.ToSignature()
 	if err != nil {
 		return err
 	}
-	if !cSig.Verify(sigHash, &verifyKey) {
+	if !cSig.Verify(sigHash, verifyKey) {
 		close(cancelChan)
 
 		// If we fail to validate their commitment signature, we'll
@@ -5485,9 +5483,7 @@ func (lc *LightningChannel) getSignedCommitTx() (*wire.MsgTx, error) {
 	localCommit := lc.channelState.LocalCommitment
 	commitTx := localCommit.CommitTx.Copy()
 
-	theirSig, err := btcec.ParseDERSignature(
-		localCommit.CommitSig, btcec.S256(),
-	)
+	theirSig, err := ecdsa.ParseDERSignature(localCommit.CommitSig)
 	if err != nil {
 		return nil, err
 	}
@@ -5906,7 +5902,7 @@ func newOutgoingHtlcResolution(signer input.Signer,
 		InputIndex:    0,
 	}
 
-	htlcSig, err := btcec.ParseDERSignature(htlc.Signature, btcec.S256())
+	htlcSig, err := ecdsa.ParseDERSignature(htlc.Signature)
 	if err != nil {
 		return nil, err
 	}
@@ -6041,7 +6037,7 @@ func newIncomingHtlcResolution(signer input.Signer,
 		InputIndex:    0,
 	}
 
-	htlcSig, err := btcec.ParseDERSignature(htlc.Signature, btcec.S256())
+	htlcSig, err := ecdsa.ParseDERSignature(htlc.Signature)
 	if err != nil {
 		return nil, err
 	}