title: in-place Update Leaflet GeoJSON feature
body: <p>I was hoping that GeoJSON.addData() would return the newly created subLayer of the GeoJSON object, but it does not. Why Do I need something like this?</p>
  
  <p>(currently using Leaflet 1.0Beta2)</p>
  
  <p>I am using Leaflet GeoJson to show live data in a GeoJSON (point, line, polygon). It is a CRUD interface (Create, Update and Delete). I receive WebSocket messages with GeoJSON data, each one with a GUID. </p>
  
  <p>I the case of a <strong>CREATE</strong> I just do a GeoJSon.AddData() to the appropriate layer.</p>
  
  <p>But for the <strong>UPDATE</strong> and <strong>DELETE</strong> I want a handle for the layer that was added to the GeoJSON so that I can update its location, or update the Geometry. addData is not giving me this handle. And it is really hard to get it from onEachFeature() or from pointToLayer()</p>
  
  <p>Currently, I do have a way that works but ugly. I have to do is search the entire layer with GeoJSon.eachLayer(fn)
  whenever an update or delete occurs. It seems a bit expensive.</p>
  
  <p>{even if Leaflet is not truly engineered for this live r/t display of data, it is working, and it seems sad if you cannot use it for watching a lot of sensor data, IoT) as we are doing.</p>
  
  <pre class="lang-js prettyprint-override"><code>this.delete = function (layerName, feature) {
      if (!(layerName in self.myLayers)) {
          alert("live.display: Missing Layer: " + layerName);
          return;
      }
      var layerInfo = Live.myLayers[layerName];
      var base = layerInfo.layer;
      var id = feature.properties.objectID;
      this.find(layerName, id, function (layer) {
          this.removeLayer(layer);
          Tools.debug(16, "live.delete:", "killed object: " + id);
      });
  }
  this.find = function (layerName, id, action) {
      var base = Live.myLayers[layerName].layer;
      base.eachLayer(function (feature) {
          if (!("objectID" in feature.feature.properties)) { return; }
          var objectID = feature.feature.properties.objectID;
          if (objectID == id) {
              action.call(base, feature);
          }
      });
  }
  </code></pre>