MSC4018: Reliable call membership

[robintown]

Rendered

[axelsimon]

This looks straightforward enough, but we might want to consider more in detail what happens if a homeserver goes offline, as this could easily become a large issue in P2P Matrix.

[toger5]

Reding through this again I think the scope of this MSC should be bigger. Introducing new endpoints for updating the membership, allows us to never grant any user permission to update m.call.member state events. Instead the HS will update those using the additional endpoints. This frees us from the necassity of using the key = user_id permission system.

We could use keys of the format: $user_id/$device_id which are all handled by the HS. This would make things much more efficient/simple (especially considering aggreagting sessions), since we dont update the whole state event with multiple memberships anymore but instead only update the data that is actually assocaited with the membership.