watchtower: use ECDH interface for watchtower session

watchtower/wtserver/server.go

@@ -8,10 +8,10 @@ import (
 	"sync"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/connmgr"
 	"github.com/btcsuite/btcutil"
+	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
 	"github.com/lightningnetwork/lnd/watchtower/wtwire"
@@ -35,7 +35,7 @@ type Config struct {
 
 	// NodeKeyECDH is the the ECDH capable wrapper of the key to be used in
 	// accepting new brontide connections.
-	NodeKeyECDH *btcec.PrivateKey
+	NodeKeyECDH keychain.SingleKeyECDH
 
 	// Listeners specifies which address to which clients may connect.
 	Listeners []net.Listener