preparing 4.4.4
* Torrent.cc, Torrent.h, TorrentTracker.cc, TorrentTracker.h:
split off TorrentTracker; add TrackerBackend class; prepare
for udp tracker implementation.
* ResMgr.cc: include stdlib.h for strtoll.
	* DHT.cc, DHT.h: add requests_in_flight node counter; clean up
	  nodes more aggresively; make just loaded nodes non-good.