IPv4-Only patch.

If torrent client requests torrent from an IPv6 link they are not
added to the [compact-format] list of IPv4 peers.
Adds a peer reaper that checks all torrents in the active cache
and removes peers from that torrent.

This is done on a separate thread; but the list of peers is protected by a RWMutex.
Peer reaper now scheduled for 10 minutes.

Seems to be working OK. Needs benchmarking.