code: function setSWRStaleEntry (service, key, value, ttl) {
    return setEntry(service, createSWRStaleKey(key), value, ttl)
  }
docs: When stale is provided the value is calculated as ttl + stale
  @param {Service} service Service instance
  @param {String} key entry key
  @param {Object} value entry value
  @param {Number|String} ttl time to live value supported by https://github.com/zeit/ms
  @returns {Promise}