title: Why bot doesn't fire messageReactionAdd event when the message was sent before the bot was online
body: <p>This is my event to check user add react:</p>
  <pre class="lang-js prettyprint-override"><code>const client = require(&quot;../index&quot;);
  
  client.on('messageReactionAdd', async (reaction, user) =&gt; {
      if (user.bot) return;
      console.log(reaction.emoji.name);
  });
  </code></pre>
  <p>It's working, but when the bot restarts, it can't fetch any message sent before?</p>
  <p>Why and how to fix it?</p>