title: Cache first implementation for get in firestore
body: <p>I want firestore to fetch data first from cache every time.
  As per Firestore documentation passing "cache" or "server" options must enable the same. Example below</p>
  
  <pre class="lang-js prettyprint-override"><code>db.collection("cities").where("capital", "==", true)
      .get("cache")
      .then(function(querySnapshot) {
          querySnapshot.forEach(function(doc) {
              // doc.data() is never undefined for query doc snapshots
              console.log(doc.id, " =&gt; ", doc.data());
          });
      })
      .catch(function(error) {
          console.log("Error getting documents: ", error);
      });
  </code></pre>
  
  <p>However whether i set "cache" or "server", query seems to try network first followed by cache.</p>