title: Mosquitto inflight_messages = 1, but got more than 1 message simultaneously
body: <p>I've set <code>max_inflight_messages</code> to 1, but after stress test, my client got multiple simultaneous messages.</p>
  
  <p>Does I misunderstood something? I thought "in-order delivery of messages" means every message should sent through PUBLISH, PUBREC, PUBREL, PUBCOMP, then next message.</p>
  
  <p>Extracted <code>mosquitto.log</code></p>
  
  <pre><code>1488858217: Sending PUBLISH to client (d0, q2, r0, m669, 'test/topic', ... (7 bytes))
  1488858217: Sending PUBLISH to client (d0, q1, r0, m670, 'test/topic', ... (7 bytes))
  1488858217: Sending PUBLISH to client (d0, q2, r0, m671, 'test/topic', ... (7 bytes))
  1488858217: Sending PUBLISH to client (d0, q2, r0, m672, 'test/topic', ... (7 bytes))
  1488858217: Sending PUBLISH to client (d0, q2, r0, m673, 'test/topic', ... (7 bytes))
  1488858217: Received PUBREC from client (Mid: 669)
  1488858217: Sending PUBREL to client (Mid: 669)
  1488858217: Received PUBACK from client (Mid: 670)
  1488858217: Received PUBREC from client (Mid: 671)
  1488858217: Sending PUBREL to client (Mid: 671)
  1488858217: Received PUBREC from client (Mid: 672)
  1488858217: Sending PUBREL to client (Mid: 672)
  1488858217: Received PUBREC from client (Mid: 673)
  1488858217: Sending PUBREL to client (Mid: 673)
  </code></pre>
  
  <p>Thanks</p>
  
  <p>This question is also post to <a href="https://github.com/eclipse/mosquitto/issues/393" rel="nofollow noreferrer">mosquitto/issues/393</a></p>