Backend changes to fetch the statistics for the database and server

nodes, along with the front-end changes for those node.

NOTE:
We've not yet added the support to show them on the front-end.

web/pgadmin/browser/server_groups/servers/templates/servers/servers.js

@@ -8,6 +8,8 @@ function($, _, S, pgAdmin, pgBrowser, alertify) {
       type: 'server',
       label: '{{ _('Server') }}',
       canDrop: true,
+      hasStatistics: true,
+      hasCollectiveStatistics: true,
       Init: function() {
 
         /* Avoid multiple registration of same menus */