Collect previous events only.

[johny-b]

Let's say we have an activity_id and a batch_id and want to fetch all events that happened to this batch.
Currently there's no clean way to do this, we might try e.g.:

async with GolemNode() as golem:
    batch = golem.batch(batch_id, activity_id)
    batch.start_collecting_events()
    await asyncio.sleep(1)
    print(batch.events)

and this is fine, except that in some rare cases single second might not be enough, and in other cases we wait too long.
Or we could:

async with GolemNode() as golem:
    batch = golem.batch(batch_id, activity_id)
    batch.start_collecting_events()
    await batch.wait()
    print(batch.events)

but this way we'll hang forever if the batch was not finished.

--> we need something like:

async with GolemNode() as golem:
    batch = golem.batch(batch_id, activity_id)
    await batch.fetch_past_events()
    print(batch.events)

This shouldn't be hard.
Probably should be implemented on golem_core.low.yagna_event_collector.YagnaEventCollector.