code: function redisSetAccountKeypair(options, keypair, callback) {
      _debug('le-store-redis.redisSetAccountKeypair', '\nkeypair:', keypair);
      var keypairId = 'keypair-' +
        crypto.createHash('sha256').update(keypair.publicKeyPem).digest('hex');
      var jsonKeypair = JSON.stringify(keypair);
  
      async.parallel([
        function(callback) {
          // write the keypair data
          client.set(keypairId, jsonKeypair, callback);
        },
        function(callback) {
          // create an index for email if one was given
          if(options.email) {
            return _createIndex('idx-e2k', options.email, keypairId, callback);
          }
          callback(null, 'NOP');
        }], function(err, results) {
          if(err) {
            return callback(err);
          }
          if(results[0] !== null && results[1] !== null) {
            callback(null, keypair);
          }
      });
    }
docs: Creates a new instance of a le-store-redis storage driver.
  
  @param {Object[]} options - options passed to storage called
  @param {string} options[].email - optional email address to associate with
  the keypair.
  @param {string} options[].accountId - optional accountId to associate with
  the keypair.
  @param {Object} keypair - a keypair provided by the node-letsencrypt
  package.
  @param {Function} callback(err, keypair) - called when an error occurs, or
  when a keypair is successfully written to the database.