This (hopefully) fixes the remaining bug where extra group calls
could be created when entering a room.
We waited for the Room event to arrive, but didn't wait for the
group call event handler to actually process the event, so it would
have depended what order the event handlers were run in.
If this doesn't fix it, it at least adds logging so we'll have more
to go on next time.
Fixes https://github.com/vector-im/element-call/issues/563
We weren't waiting for rooms to arrive down the sync stream after
joining them but before trying to use them.
More regression details in linked issue.
Fixes https://github.com/vector-im/element-call/issues/477
