Correctly handle redacted state events

[jplatte]

We currently filter out redacted state events in some places and only handle unredacted ones. This is incorrect: Redacted state events still affect the room state.

Left to do after #627:

• Review remaining uses of FooEvent::Original
• Add fetching of events by ID to the state store, then apply incoming redactions to stored events
  • State events
  • Timeline events