multi: move BackupState and RegisterChannel to Manager

This commit moves over the last two methods, `RegisterChannel` and
`BackupState` from the `Client` to the `Manager` interface. With this
change, we no longer need to pass around the individual clients around
and now only need to pass the manager around.

To do this change, all the goroutines that handle channel closes,
closable sessions needed to be moved to the Manager and so a large part
of this commit is just moving this code from the TowerClient to the
Manager.

peer/brontide.go

@@ -268,12 +268,8 @@ type Config struct {
 	// HtlcNotifier is used when creating a ChannelLink.
 	HtlcNotifier *htlcswitch.HtlcNotifier
 
-	// TowerClient is used by legacy channels to backup revoked states.
-	TowerClient wtclient.Client
-
-	// AnchorTowerClient is used by anchor channels to backup revoked
-	// states.
-	AnchorTowerClient wtclient.Client
+	// TowerClient is used to backup revoked states.
+	TowerClient wtclient.TowerClientManager
 
 	// DisconnectPeer is used to disconnect this peer if the cooperative close
 	// process fails.
@@ -1040,14 +1036,8 @@ func (p *Brontide) addLink(chanPoint *wire.OutPoint,
 		return p.cfg.ChainArb.NotifyContractUpdate(*chanPoint, update)
 	}
 
-	chanType := lnChan.State().ChanType
-
-	// Select the appropriate tower client based on the channel type. It's
-	// okay if the clients are disabled altogether and these values are nil,
-	// as the link will check for nilness before using either.
-	var towerClient htlcswitch.TowerClient
-	switch {
-	case chanType.IsTaproot():
+	var towerClient wtclient.TowerClientManager
+	if lnChan.ChanType().IsTaproot() {
 		// Leave the tower client as nil for now until the tower client
 		// has support for taproot channels.
 		//
@@ -1060,9 +1050,7 @@ func (p *Brontide) addLink(chanPoint *wire.OutPoint,
 				"are not yet taproot channel compatible",
 				chanPoint)
 		}
-	case chanType.HasAnchors():
-		towerClient = p.cfg.AnchorTowerClient
-	default:
+	} else {
 		towerClient = p.cfg.TowerClient
 	}