libtracker-miner: clear directories queue before TrackerCrawler::finished
libtracker-miner: clear directories queue before TrackerCrawler::finished

This is so we can call tracker_crawler_start() within the ::finished
handler, instead of on an idle.

hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker