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

channeldb/db_test.go

@@ -10,10 +10,10 @@ import (
 	"reflect"
 	"testing"
 
-	"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/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -300,15 +300,13 @@ func genRandomChannelShell() (*ChannelShell, error) {
 		return nil, err
 	}
 
-	_, pub := btcec.PrivKeyFromBytes(btcec.S256(), testPriv[:])
+	_, pub := btcec.PrivKeyFromBytes(testPriv[:])
 
 	var chanPoint wire.OutPoint
 	if _, err := rand.Read(chanPoint.Hash[:]); err != nil {
 		return nil, err
 	}
 
-	pub.Curve = nil
-
 	chanPoint.Index = uint32(rand.Intn(math.MaxUint16))
 
 	chanStatus := ChanStatusDefault | ChanStatusRestored