Bugfix: don't return extra member count or e2ee device updates from sync

[olivia-fl]

Previously, we were returning redundant member count updates or encrypted device updates from the /sync endpoint in some cases. The extra member count updates are spec-compliant, but unnecessary, while the extra encrypted device updates violate the spec.

The refactor necessary to fix this bug is also necessary to support filtering on state events in sync.

Details

Joined room incremental sync needs to examine state events for four purposes:

1. determining whether we need to return an update to room member counts
2. determining the set of left/joined devices for encrypted rooms (returned in device_lists)
3. returning state events to the client (in rooms.joined.*.state)
4. tracking which member events we have sent to the client, so they can be omitted on future requests when lazy-loading is enabled.

The state events that we need to examine for the first two cases is member events in the delta between since and the end of timeline. For the second two cases, we need the delta between since and the start of timeline, plus contextual member events for any senders that occur in timeline. The second list is subject to filtering, while the first is not.

Before this change, we were using the same set of state events that we are returning to the client (cases 3/4) to do the analysis for cases 1/2. In a compliant implementation, this would result in us missing some relevant member events in 1/2 in addition to seeing redundant member events. In current conduwuit this is not the case because the set of events that we return to the client is always a superset of the set that is needed for cases 1/2. This is because we don't support filtering, and we have an existing bug where we are returning the delta between since and the end of timeline rather than the start.

Fixing this is necessary to implement filtering because otherwise we would start missing some member events for member count or encrypted device updates if the relevant member events are rejected by the filter. This would be much worse than our current behavior.