Support for repeated reduce phase

[tomachalek]

ApplyItems -> Map -> Reduce -> ... -> Reduce -> Finish

This may be useful when processing recursive or graph-like structures (e.g. crawling the Web).

"static" variant

reduce(function (key, values) {
   // 1st run
});
reduce(function (key, values) {
   // 2nd run
});

"dynamic" variant

reduce(function (key, values) {
   // the function is called again and again as long as reduceLoop returns true
});

reduceLoop(function (???) {
  if (done) {
    return false; // => finish

  }  else {
    return true;
  }
});