Освобождаем QuadTree от лишнего кода и зависимостей от OpenLayers.
Разносим кластеризацию по тредам с передачей данных "по ссылке".
Fix some bugs in parallel clustering - now it really rocks!

1. Add listener to each worker only 1 time - this greatly improves
perfomance.
2. Add to Cluster strategy `dataExtent' similar to BBOX strategy,
defined with ratio (ratio of data extent to current map extent) - and
cluster only those features that are contained in dataExtent.
Add `ratio' property to Cluster strategy.