Load markers cluster layer by chunks

In order to avoid locking the browser for a long time.
when there are a lot of markers in the map:

https://github.com/Leaflet/Leaflet.markercluster#handling-lots-of-markers

app/assets/javascripts/map.js

@@ -23,7 +23,7 @@
       editable = $(element).data("marker-editable");
       markerClustering = $(element).data("marker-clustering");
       if (markerClustering) {
-        markers = L.markerClusterGroup();
+        markers = L.markerClusterGroup({ chunkedLoading: true });
       } else {
         markers = L.layerGroup();
       }