peer: define Config and LinkUpdater interface

This commit defines a Config for initializing a peer and the
LinkUpdater interface.

peer/interfaces.go

@@ -0,0 +1,11 @@
+package peer
+
+import "github.com/lightningnetwork/lnd/lnwire"
+
+// LinkUpdater is an interface implemented by most messages in BOLT 2 that are
+// allowed to update the channel state.
+type LinkUpdater interface {
+	// TargetChanID returns the channel id of the link for which this message
+	// is intended.
+	TargetChanID() lnwire.ChannelID
+}