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
```
Use fully-qualified `PersistedEventPosition` (`instance_name` and `stream_ordering`) when returning `RoomsForUser` to facilitate proper comparisons and `RoomStreamToken` generation.
Spawning from https://github.com/element-hq/synapse/pull/17187 where we want to utilize this change
Before:
```
$ SYNAPSE_POSTGRES=1 SYNAPSE_POSTGRES_USER=postgres SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.replication.storage.test_events
[...]
Traceback (most recent call last):
File "pypoetry/virtualenvs/matrix-synapse-xCtC9ulO-py3.12/lib/python3.12/site-packages/twisted/trial/runner.py", line 711, in loadByName
return self.suiteFactory([self.findByName(name, recurse=recurse)])
File "pypoetry/virtualenvs/matrix-synapse-xCtC9ulO-py3.12/lib/python3.12/site-packages/twisted/trial/runner.py", line 474, in findByName
obj = reflect.namedModule(searchName)
File "pypoetry/virtualenvs/matrix-synapse-xCtC9ulO-py3.12/lib/python3.12/site-packages/twisted/python/reflect.py", line 156, in namedModule
topLevel = __import__(name)
File "synapse/tests/replication/storage/test_events.py", line 33, in <module>
from synapse.handlers.room import RoomEventSource
File "synapse/synapse/handlers/room.py", line 74, in <module>
from synapse.rest.admin._base import assert_user_is_admin
File "synapse/synapse/rest/__init__.py", line 24, in <module>
from synapse.rest import admin
File "synapse/synapse/rest/admin/__init__.py", line 41, in <module>
from synapse.handlers.pagination import PURGE_HISTORY_ACTION_NAME
File "synapse/synapse/handlers/pagination.py", line 30, in <module>
from synapse.handlers.room import ShutdownRoomParams, ShutdownRoomResponse
builtins.ImportError: cannot import name 'ShutdownRoomParams' from partially initialized module 'synapse.handlers.room' (most likely due to a circular import) (synapse/synapse/handlers/room.py)
```
