code: function diffRecords(localRecords, newRecords) {
    var recordsDiff = {};
    _.each(newRecords, function(record, uid) {
      if (localRecords[uid]) {
        if (localRecords[uid].hash !== record.hash) {
          record.op = "update";
          recordsDiff[uid] = record;
        }
      } else {
        record.op = "update";
        recordsDiff[uid] = record;
      }
    });
    _.each(localRecords, function(record, uid) {
      if (!newRecords[uid]) {
        record.op = "delete";
        recordsDiff[uid] = record;
      }
    });
    return recordsDiff;
  }
docs: Compute the diff between the 2 objects based on the hash value
  @param {Object} localRecords
  @param {Object} newRecords
  @returns an object that contains the diff