constructor (client, announceUrl) {
    super(client, announceUrl)
    debug('new udp tracker %s', announceUrl)

    this.cleanupFns = []
    this.maybeDestroyCleanup = null
  }
UDP torrent tracker client (for an individual tracker)

@param {Client} client       parent bittorrent tracker client
@param {string} announceUrl  announce url of tracker
@param {Object} opts         options object