Fix honoring "throttle.min_peers*" settings in rtorrent

[chros73]

Fix honoring throttle.min_peers.normal and throttle.min_peers.seed settings in rtorrent:

• the bug was introduced by this commit (introduced in v0.13.4)
• it never clears the flag_requesting flag for private trackers in torrent/tracker_controller.cc

Due to this bug, handling interval of announce tracker request is broken:

• it never use interval but min interval instead

Required behavior:

• it will use tracker interval if:
  • the client has more than min_peers - (peer_list_size / 2) connections for a download
  • or Peer Exchange is enabled and gives connections for a download
• it will use tracker min interval otherwise:
  • the client has less than min_peers - (peer_list_size / 2) connections for a download and Peer Exchange isn't enabled

Default values for throttle.min_peers* settings: 100

Fixes: #167, rakshasa/rtorrent#386
Refers to: #168

[coveralls]

Coverage decreased (-0.01%) to 18.027% when pulling ee8d5be on chros73:fix_min_peers_settings into c167c5a on rakshasa:master.