Show how to keep asynchronous responses in order

The REP socket and the WORKER socket must reply (or acknowledge, for
the worker) in the order in which they received messages. If replying
involves some asynchronous task, the choice is

 1. only read one thing at a time, and wait until the callback is
 invoked to read another;
 2. read as many as you can, and reorder the responses when they come
 back.

This example shows how to do the latter.
Refer to the ordering example in the README