lib/sync-cache.js
function del(key, cb) {
  if (!syncConfig.useCache || !redisClient) {
    return cb && cb();
  }
  return redisClient.del(key, cb);
}
function del(key, cb) {
  if (!syncConfig.useCache || !redisClient) {
    return cb && cb();
  }
  return redisClient.del(key, cb);
}
Delete data from cache
@param {String} key
@param {Function} cb