Virtualchain tx filter function:
    * only take txs whose OP_RETURN payload starts with 'id'

def blockstack_tx_filter( tx ):
    """
    Virtualchain tx filter function:
    * only take txs whose OP_RETURN payload starts with 'id'
    """
    if not 'nulldata' in tx:
        return False
    
    if tx['nulldata'] is None:
        return False

    payload = binascii.unhexlify( tx['nulldata'] )
    if payload.startswith(blockstack_magic_bytes()):
        return True

    else:
        return False