Enumerates over individual items of a request, paging the responses if
necessary.

@api experimental
@since v1.4.0
function eachItem(callback) {
    function wrappedCallback(err, data) {
      if (err) return callback(err, null);
      if (data === null) return callback(null, null);

      var config = this.request.service.paginationConfig(this.request.operation);
      var resultKey = config.resultKey;
      if (Array.isArray(resultKey)) resultKey = resultKey[0];
      var results = AWS.util.jamespath.query(resultKey, data);
      AWS.util.arrayEach(results, function(result) {
        AWS.util.arrayEach(result, function(item) { callback(null, item); });
      });
    }

    this.eachPage(wrappedCallback);
  }