Save the sync update to the `fhsync-<datasetId>-updates` collection

@param {String} datasetId
@param {storage~Acknowledgement} acknowledgementFields
@param {Function} callback
function doSaveUpdate(datasetId, acknowledgementFields, callback) {
  debug('[%s] doSaveUpdate acknowledgementFields = %j' , datasetId, acknowledgementFields);
  var updatesCollection = mongoClient.collection(getDatasetUpdatesCollectionName(datasetId));
  updatesCollection.findOneAndUpdate({cuid: acknowledgementFields.cuid, hash: acknowledgementFields.hash}, {'$set': acknowledgementFields}, {upsert: true, returnOriginal: false}, function(err, updateResult) {
    if (err) {
      debugError('[%s] Failed to doSaveUpdate due to error %s :: acknowledgementFields = %j' ,datasetId,err,acknowledgementFields);
      return callback(err);
    }
    return callback(null, updateResult.value);
  });
}