'Persists the `Transaction` we are about to send and works out the
correct value for the `prev_ids` key.
Returns:
Deferred'

@defer.inlineCallbacks
@log_function
def prepare_to_send(self, transaction):

    transaction.prev_ids = (yield self.store.prep_send_transaction(transaction.transaction_id, transaction.destination, transaction.origin_server_ts))
