The `new_token` parameter of `Notifier.on_new_event` seems to be misunderstood

[`Notifier.on_new_event`](https://github.com/element-hq/synapse/blob/5ed830b3b4c74c89d876cc07756c5d98a100cbed/synapse/notifier.py#L500-L554) has a `new_token` parameter.

To me, it looks like that's supposed to get the stream's new position passed (as you'd get from `.get_current_token()` on an ID generator).

In actuality, for some stream types (e.g. thread subscriptions), we are passing the `stream_id` from newly-inserted rows. In a multi-writer setup, new rows don't immediately become the current stream position.

It seems it only matters for a small handful of stream types, so the relatively-new streams getting this wrong might not have any real issues from this misunderstanding.

- [ ] identify and fix up the mistakes
- [ ] there might be documentation to correct