removed old imports
Refactoring StatisticsTracker for better modularity:
create default Report objecs through BeanFactory so that Reports can leverage dep injection for gaining
access to CrawlController and Frontier (or whatever). StatisticsTracker ref is also injected (2nd parameter
to writeReport() is unused).
moved/TODO-annotated a few StatisticsTracker methods specific to particular Report.
moved StatisticsTracker#threadCount() to CrawlJob, and made it private (for now).
StatisticsTracker still has threadCount() method, but it's @Deprecated.