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
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;
}