Fix failing /sync tests

*This issue was originally created by [**@kegsay**](https://github.com/kegsay) at .*

They should be passing but for some reason are not.

Sytests:
```
× Newly joined room has correct timeline in incremental sync
× Newly joined room includes presence in incremental sync
× When user joins a room the state is included in the next sync
× When user joins a room the state is included in a gapped sync
× When user joins and leaves a room in the same batch, the full state is still included in the next sync
× Current state appears in timeline in private history with many messages after
```

Start by running the six listed Sytests for incremental sync and inspect the /sync implementation and its handling of joined-room state, presence, gapped syncs, and private history. Done means all six currently failing Sytests pass without regressions.

Written by the indexing model from the issue text.