lib/sync-cache.js
function set(key, value, cb) {
  if (!syncConfig.useCache || !redisClient) {
    return cb && cb();
  }
  return redisClient.set(key, value, cb);
}
function set(key, value, cb) {
  if (!syncConfig.useCache || !redisClient) {
    return cb && cb();
  }
  return redisClient.set(key, value, cb);
}
Save data to the cache
@param {String} key
@param {String} value
@param {Function} cb