Sliding sync: handle room subscriptions changing required_state.

[erikjohnston]

Synapse doesn't currently take into account changes to required_state in room config like it does for the timeline_limit. We should track the previous required_state and if it expands we should send down the current state for the new keys.

Motivating example is so that for room subscriptions the app can ask for more state, like room topic etc, without having to get it for all rooms.

This currently breaks things like topics and pinned events in EXI.

c.f.

synapse/synapse/handlers/sliding_sync/__init__.py

Line 558 in f6c2b0e

# TODO: Check for changes in `required_state``