using AS OF SYSTEM TIME

Is there an any support for [`AS OF SYSTEM TIME`](https://www.cockroachlabs.com/docs/stable/as-of-system-time.html) in either the transaction variant:

```sql
BEGIN AS OF SYSTEM TIME '2019-04-09 18:02:52.0+00:00';
```

or the query variant:

```sql
SELECT * FROM t, u, v AS OF SYSTEM TIME '-4h';
```

?

It seems to me that there is currently no way to use this cockroach feature from within django. The code emitting `BEGIN` statements seems to be berried 3 dependencies down the line within [`psycopg.BaseConnection._get_tx_start_command`](https://github.com/psycopg/psycopg/blob/master/psycopg/psycopg/connection.py#L497-L515), so I am not sure how to approach this elegantly.

The API could like like this for transactions (extended `django.db.transaction.Atomic`/`.atomic`):

```python
from django_cockroachdb.transaction import atomic

with atomic(as_of_system_time="-4h"):
pass
```

or like this for queries (modified `QuerySet` class):

```python
Poll.objects.all().as_of_system_time("-4h")
```

Are there any plans to add this?