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, https://github.com/rakshasa/rtorrent/issues/386 Refers to: #168

[coveralls]

Coverage decreased (-0.01%) to 18.027% when pulling ee8d5be19c1ec77b5989f182dbdff001f81763ec on chros73:fix_min_peers_settings into c167c5a9e0bcf0df23ae5efd91396aae0e37eb87 on rakshasa:master.