Make the logic of publishing update tx more robust

[linked0]

A flash node sends the latest update tx right after a block containing an update tx is externalized. But there might be some issue that the validator referenced by the node has not yet accepted the block so there might be no UTXO that is referenced by inputs of a new update tx. So we should change the logic that can have a relayer or check that the referenced validator has the same height as the node. This issue was found out when solving issue #2425.

    public void onUpdateTxExternalized (in Transaction tx, in uint utxo_idx, in bool is_last)
    {
        this.state = ChannelState.StartedUnilateralClose;
        this.onChannelNotify(this.own_pk, this.conf.chan_id, this.state,
            ErrorCode.None);


        this.last_externalized_update_utxo = UTXO.getHash(tx.hashFull(), utxo_idx);
        if (is_last)
        {
            this.state = ChannelState.WaitingOnSettlement;
            this.onChannelNotify(this.own_pk, this.conf.chan_id, this.state,
                ErrorCode.None);
            this.update_ext_height = this.height;
        }
        else
        {
            this.publishUpdateTx(this.channel_updates[$ - 1]);
        }
    }