wtclient: spin session key indices if required

If the tower returns CreateSessionCodeAlreadyExists in response to the
CreateSession message from the client, then skip forward a few key
indices until we find one that the server does not return the error
for. This will allow a client to recover after a data loss incident.

watchtower/wtclient/errors.go

@@ -34,4 +34,9 @@ var (
 	// revoked state because the channel had not been previously registered
 	// with the client.
 	ErrUnregisteredChannel = errors.New("channel is not registered")
+
+	// ErrSessionKeyAlreadyUsed indicates that the client attempted to
+	// create a new session with a tower with a session key that has already
+	// been used in the past.
+	ErrSessionKeyAlreadyUsed = errors.New("session key already used")
 )