htlcswitch: add channel link config

Step №1 in making htlcManager (aka channelLink) testable:
Start use config which will allow as mock/stub external subsystems.

htlcswitch/mock.go

@@ -10,6 +10,7 @@ import (
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/roasbeef/btcutil"
+	"github.com/lightningnetwork/lnd/lnwallet"
 )
 
 type mockServer struct {
@@ -152,6 +153,10 @@ func (s *mockServer) Disconnect() {
 	s.t.Fatalf("server %v was disconnected", s.name)
 }
 
+func (s *mockServer) WipeChannel(*lnwallet.LightningChannel) error {
+	return nil
+}
+
 func (s *mockServer) Stop() {
 	if !atomic.CompareAndSwapInt32(&s.shutdown, 0, 1) {
 		return