code: function redisCheckAccountKeypair(options, callback) {
      _debug('le-store-redis.redisCheckAccountKeypair:',
        options.email, options.accountId);
  
      if(options.email) {
        return _getByIndex('idx-e2k', options.email, callback);
      } else if(options.accountId) {
        return _getByIndex('idx-a2k', options.accountId, callback);
      }
  
      callback(new Error('le-store-redis.redisCheckAccountKeypair ' +
        'lookup requires options.email or options.accountId.'));
    }
docs: Retrieves a keypair associated with an account from the database.
  
  @param {Object[]} options - options passed to storage call
  @param {string} options[].email - email address associated with
  registration.
  @param {string} options[].accountId - account ID as returned from
  redisSetAccount()
  @param {Function} callback(err, keypair) - called after storage attempt,
  keypair will be null if it was not found.