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.

lnrpc/wtclientrpc/config.go

@@ -15,14 +15,6 @@ type Config struct {
 	// Active indicates if the watchtower client is enabled.
 	Active bool
 
-	// Client is the backing watchtower client that we'll interact with
-	// through the watchtower RPC subserver.
-	Client wtclient.Client
-
-	// AnchorClient is the backing watchtower client for anchor channels that
-	// we'll interact through the watchtower RPC subserver.
-	AnchorClient wtclient.Client
-
 	// ClientMgr is a tower client manager that manages a set of tower
 	// clients.
 	ClientMgr wtclient.TowerClientManager