title: What is the once listener equivalent in angularfire2?
body: <p>I was wondering, could we say that:
  <code>this.af.database.object('whatever').first().toPromise()</code>
  is equivalent to :
  <code>firebase.database().ref('whatever').once('value', snapshot=&gt;{})</code></p>
  
  <p>I red that firebase caches the data (in memory) for as long as there is an active listener for that data.
  So in the case of the once listenner, the data is cleared from the cache. 
  Is that the same behavior when I use the first().toPromise(), so does it detached the listenner immediately when the data is received ? </p>
  
  <p>I ask you this question because I often use async and I don't want to lose in performance.</p>