Before, the problem scenario would get caught in one of the assertions because
we expect the to_token <= membership_snapshot_token or vice-versa but it's
possible the tokens are intertwined and neither is ahead of each other.
Especially since the `instance_map` in `membership_snapshot_token` is made up
from the `stream_ordering` of membership events at various stream positions
and processed on different instances (not current stream positions).
We get into trouble when stream positions are lagging between workers and our
now/`to_token` doesn't cleanly compare to `membership_snapshot_token`.
What we really want to assert is that the `to_token` <= the stream positions
at the time we asked for the room membership snapshot. Since
`get_rooms_for_local_user_where_membership_is()` doesn't return that
information, the closest we can get is to get the stream positions before we
ask for the room membership snapshot and consider that good enough to compare
against.
The new implementation catches the problem with an assert
but I think it's possible to make it work as well.
```
SYNAPSE_POSTGRES=1 SYNAPSE_POSTGRES_USER=postgres SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.handlers.test_sliding_sync.GetSyncRoomIdsForUserEventShardTestCase
```
