title: limit_req differ zones by time of day
body: <p>I want to limit requests to my service depending on the time of day.<br>
  It should be 4 r/s by night (21:00 - 06:59) and 1 r/s by day (07:00-20:59).<br>
  I could have 2 different <code>req_limit_zone</code> like  </p>
  
  <pre><code>limit_req_zone $server_name zone=day:1m rate=1r/s;
  limit_req_zone $server_name zone=night:1m rate=4r/s;
  </code></pre>
  
  <p>but how can I differ them in <code>limit_req</code> depending on the time of day?  </p>
  
  <p>The service behind nginx is an API wrapper for another service with limits described above. My service is written with <code>Flask-RESTful</code>, runs by <code>uWSGI</code> with a few processes, so it would be pretty painful to implement limiting logic and sync it between processes with something like <code>redis</code>. </p>
  
  <p>Is it possible to configure nginx this way?<br>
  If it's not, are there any common workarounds? How do other services solve this task?<br>
  To be more specific, it should be a leaking bucket, so clients just wait for an answer without any errors like HTTP 429 Too Many Requests.</p>