start (opts) {
    opts = this._defaultAnnounceOpts(opts)
    opts.event = 'started'
    debug('send `start` %o', opts)
    this._announce(opts)

    // start announcing on intervals
    this._trackers.forEach(tracker => {
      tracker.setInterval()
    })
  }
Send a `start` announce to the trackers.
@param {Object} opts
@param {number=} opts.uploaded
@param {number=} opts.downloaded
@param {number=} opts.left (if not set, calculated automatically)