title: Ag-Grid rowDataChanged not firing properly
body: <p>Here is my <code>onDataChanged()</code> event. It's plugged into my Ag-Grid HTML. It does fire (3 times) but each time it only thinks there's 1 row being displayed. I'm using the serverSide row model and data coming in from the server is a bit slow, so I think that's the problem. I need to have this event fire when the data is changed though, so I can perform some actions when I have a full list of data. At the moment, again, it only ever thinks there's 1 row being displayed when I see that there are 20 in the list.</p>
  
  <pre><code>     onDataChanged: function(event) {
          var count;
          console.log("data changed");
          count = this.gridOptions.api.getDisplayedRowCount();
          console.log(count);
        }
  // only ever outputs "1" even though I see 20+ items in the list
  </code></pre>
  
  <p>Because I'm using the serverSide row model, I'm using a serverDatasource to populate the data. Is there another way to detect when data has been changed? Thank you</p>