title: zeromq pattern: pub/sub with guaranteed delivery
body: <p>I'd like to fan out messages and be sure that any connected socket has received them, if present.</p>
  
  <p>So is there a pattern to achieve the following:</p>
  
  <ul>
  <li>PUB socket shall block on watermark if there is one or more SUB connected, i.e. all of the SUB sockets have received the message.</li>
  <li>PUB socket shall drop on watermark if there is no SUB connected.</li>
  </ul>
  
  <p>I use <a href="https://github.com/jonnydee/nzmqt" rel="nofollow">nzmqt</a> within Qt5 on OSX/Linux, if this is helpful or the mentioned problem could be solved in this special case.</p>