'Persists an outgoing transaction and calculates the values for the
previous transaction id list.
This should be called before sending the transaction so that it has the
correct value for the `prev_ids` key.
Args:
transaction_id (str)
destination (str)
origin_server_ts (int)
Returns:
list: A list of previous transaction ids.'

def prep_send_transaction(self, transaction_id, destination, origin_server_ts):

    return defer.succeed([])
