DupeFilter: add settings for verbose logging and filtered requests stats

Too many duplicate requests is often not a good sign (buggy spider or misbehaving website) -- not for `CrawlSpider` obviously.

Here I propose adding 1 setting for the default `RFPDupeFilter`class:

* `DUPEFILTER_DEBUG`: to log all duplicate requests and not only the first one

This patch also adds a stats counter with key `dupefilter/filtered` to count the number of duplicate requests.
This counter is always enabled.

Question: is `spider.crawler.stats` the proper way to access the crawler's stats collector? is it safe?