fix(adkg): concurrency issue when waiting for RBCs

[azixus]

Two issues are fixed in that PR.

The first is to wait for RBC instances to completes once all ABAs have been completed. Not a bug, but otherwise the ABAs may run for longer, which results in unnecessary communications.

The second issue, was more problematic, but with low probability of occurring. Essentially, before executing the final RBC, we were waiting as follows:

loop {
  if rbc_input \subset current_acss_completed
    break;
  else
    await register_notification();
}

With the way the notification system works, a permit is only stored if a thread is already waiting, otherwise, no permit was issued. This means that if the notification occurred right after the condition, and before we actually await on the notification, it would be missed.

I fixed it by registering for the notification prior to checking the condition:

loop {
  notification = register_notification();
  if rbc_input \subset current_acss_completed
    break;
  else
    await notification;
}

[AnomalRoil]

LGTM, just a little question and a reminder.