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.

htlcswitch/link.go

@@ -416,7 +416,9 @@ func (l *channelLink) Start() error {
 	// If the config supplied watchtower client, ensure the channel is
 	// registered before trying to use it during operation.
 	if l.cfg.TowerClient != nil {
-		err := l.cfg.TowerClient.RegisterChannel(l.ChanID())
+		err := l.cfg.TowerClient.RegisterChannel(
+			l.ChanID(), l.channel.State().ChanType,
+		)
 		if err != nil {
 			return err
 		}