Refactor GroupCall participant management
This refactoring brings a number of improvements to GroupCall, which I've unfortunately had to combine into a single commit due to coupling:
- Moves the expiration timestamp field on call membership state to be per-device
- Makes the participants of a group call visible without having to enter the call yourself
- Enables users to join group calls from multiple devices
- Identifies active speakers by their call feed, rather than just their user ID
- Plays nicely with clients that can be in multiple calls in a room at once
- Fixes a memory leak caused by the call retry loop never stopping
- Changes GroupCall to update its state synchronously, and write back to room state asynchronously
- This was already sort of halfway being done, but now we'd be committing to it
- Generally improves the robustness of the state machine
- It means that group call joins will appear instant, in a sense
For many reasons, this is a breaking change.
