it removed support for functions being simultaneous async and have done parameter in callback

Drop end to end signature. The impersonation attack vector is deemed out of
scope since insiders are already part of the meeting.

Signs and verifies frames. For now this is done on every frame.
This is not the long-term goal and there needs to be a better strategy.

and make them a bit more concise. No functional changes

Both for just encode function as well as an end-to-end test.