Don't break causality when rolling back state
We need to use the *previous* state when rolling back or else
causality breaks. Consider the messages:
- m.room.member : Alice
- Alice: 1
- Alice: 2
- m.room.member : Alice -> Bob
- Bob: 3
- Bob: 4
If we roll back 4 messages (to Alice: 2), we want the rolled
back m.room.member value to be "Alice" and NOT Bob. This
means we need to look at the previous state of the m.room.member
event and not the current state.
