Tracker improvements

Some improvements:

* Peers were being timed-out prematurely; announce requests were not touching the persisted representation of the peer
* The announcing peer is now excluded from the list of peers returned in the tracker response. Including the announcing peer could otherwise potentially result in a client requesting pieces from itself.
* Reduced the announce interval to 30 seconds.

Based on [Chris Shoff's fork](https://github.com/chrisshoff/node-bittorrent-tracker).