server: make onUdpRequest() part of public API
server: make onUdpRequest() part of public API

As per https://github.com/feross/bittorrent-tracker/pull/48#issuecomment-66893626

maxogden/bittorrent-tracker,jhiesey/bittorrent-tracker,feross/bittorrent-tracker,prozacchiwawa/bittorrent-tracker,precisit/bittorrent-tracker,LinusU/bittorrent-tracker,feross/bittorrent-tracker,jhiesey/bittorrent-tracker