code: function(cursor, callback) {
      groupedDelta = {};
  
      async.series([
        (cb) => {
          logger.info('Fetching delta', {category: 'sync-remote-delta'});
  
          fetchDelta({cursor}, (err, newCursor) => {
            if(err) return cb(err);
  
            cb(null, newCursor);
          });
        },
        (cb) => {
          logger.info('Applying collections which need to be redownloaded', {category: 'sync-remote-delta'});
  
          syncDb.find({$and: [{directory: true}, {downloadOriginal: true}]}, (err, syncedNodes) => {
            if(err) return cb(err);
            if(!syncedNodes) return cb(null);
  
            async.map(syncedNodes, (syncedNode, mapCb) => {
              if(!syncedNode.remoteId) mapCb(null);
  
              fetchDelta({id: syncedNode.remoteId}, mapCb);
            }, cb);
          });
        }
      ], (err, results) => {
        logger.info('getDelta ended', {category: 'sync-remote-delta'});
  
        callback(err, results[0]);
      });
    }
docs: fetches the delta from the server,
  
  @param {string|undefined} cursor - the cursor to get the delta from
  @param {Function} callback - callback function
  @returns {void} - no return value