fix typo
Convert the mutex in message_oriented_connection that protects two concurrent calls to send_message() into a simple assert/log verifying that it is never called concurrently (peer_connection already assures this will be the case).  Change the pending_message queue in peer_connection into a list-based queue to make it safer to add to the queue while send_message is in progress (send_message didn't access the queue after yielding so this never caused a bug, but it still feels wrong)