Sliding sync does not correctly do lazy loaded members for incremental sync

[erikjohnston]

For incremental syncs of rooms, lazy loaded members means we must send down the membership events for all senders of events in the timeline section of a room if those membership events have not been previously been sent down.

Currently we only correctly do this for initial syncs.

In sync v2 we do this by maintaining in-memory caches of which membership events we've sent down which sync streams.