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

watchtower/wtdb/client_db.go

@@ -7,7 +7,7 @@ import (
 	"math"
 	"net"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/blob"

watchtower/wtdb/client_db_test.go

@@ -10,7 +10,7 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/blob"

watchtower/wtdb/codec_test.go

@@ -10,13 +10,13 @@ import (
 	"testing"
 	"testing/quick"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/tor"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
 )
 
 func randPubKey() (*btcec.PublicKey, error) {
-	priv, err := btcec.NewPrivateKey(btcec.S256())
+	priv, err := btcec.NewPrivateKey()
 	if err != nil {
 		return nil, err
 	}

watchtower/wtdb/session_id.go

@@ -3,7 +3,7 @@ package wtdb
 import (
 	"encoding/hex"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 )
 
 // SessionIDSize is 33-bytes; it is a serialized, compressed public key.

watchtower/wtdb/tower.go

@@ -6,7 +6,7 @@ import (
 	"io"
 	"net"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 )