Process messages with the room state corresponding to the event instead of the room state at processing time

[maxidorius]

In case of catching up or in a burst of event, we end up sending the notification to people who joined after the said event, or not sending to people who left after the event.

We should fetch the room state at the time of the event and process that instead.

[maxidorius]

Following a conversation with devs (start - end), there is no clean or good way to do this unless a copy of the room state is kept on the AS side.