Using gather inside transaction may leads to errors

[kalombos]

async with db.aio_atomic():
    await asyncio.gather(t1(), t2(), t3())

It seems fine if all tasks inside gather have no transaction but if t1 and t2 have aio_atomic statement inside it leads to mess of savepoints and errors.

[spumer]

It seems fine if all tasks inside gather have no transaction

any exception in SQL STATEMENT will interrupt outer transaction and cause errors in other tasks too

[kalombos]

That's right. Thank you @spumer. As you said the solution is to add kind of forbid_subtask param to aio_atomic which forbids using of aio_atomic in subtasks.

[kalombos]

related #215 #123