htlcswitch: avoid leaking peer interface from link

Here we notice that the only use of the Peer call on the link is
to find out what the peer's pubkey is. To avoid leaking handles to
IO actions outside the interface we reduce the surface area to
just return the peer's public key.

htlcswitch/interfaces.go

@@ -7,7 +7,6 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/invoices"
-	"github.com/lightningnetwork/lnd/lnpeer"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -262,9 +261,9 @@ type ChannelLink interface {
 	// total sent/received milli-satoshis.
 	Stats() (uint64, lnwire.MilliSatoshi, lnwire.MilliSatoshi)
 
-	// Peer returns the representation of remote peer with which we have
-	// the channel link opened.
-	Peer() lnpeer.Peer
+	// Peer returns the serialized public key of remote peer with which we
+	// have the channel link opened.
+	PeerPubKey() [33]byte
 
 	// AttachMailBox delivers an active MailBox to the link. The MailBox may
 	// have buffered messages.