Order of PreKey and Encrypted message

[EbnerAndreas]

The partner sends us the prekey and the m.room.enrypted message of the first room message almost at the same time.
The sync() reads both events in one HTTP response and puts both into the corresponding event loops.
Due to the fact that we have an asynchronous behavior it can happen that we are trying to decrypt the message before the prekey message is handled. Therefore, at the moment, we have this workaround:

        if not meg_ses:
            await asyncio.sleep(5)
            meg_ses = self.room_keys.get((message[1], sender_device_id, session_id))

Maybe a better solution can be found to synchronize the events.