Fix #129: use transaction ID from connection's own transaction.

See #129 for rationale.
A quick check
```sh
$ git grep -l rollback_transaction
gcloud/datastore/connection.py
gcloud/datastore/test_connection.py
gcloud/datastore/test_transaction.py
gcloud/datastore/transaction.py
```
shows that `rollback_transcation` is used in `gcloud/datastore/transaction.py`.