multi: extend lnpeer.Peer interface with Disconnect function

This will be used in the gossiper to disconnect from peers if their
ban score passes the ban threshold.

funding/manager_test.go

@@ -283,6 +283,8 @@ type testNode struct {
 
 var _ lnpeer.Peer = (*testNode)(nil)
 
+func (n *testNode) Disconnect(err error) {}
+
 func (n *testNode) IdentityKey() *btcec.PublicKey {
 	return n.addr.IdentityKey
 }