rescanFiles (cb) {
    if (this.destroyed) throw new Error('torrent is destroyed')
    if (!cb) cb = noop

    this._verifyPiecesUsingHash(this.pieces, (err) => {
      if (err) {
        this._destroy(err)
        return cb(err)
      }

      this._checkDone()
      cb(null)
    })
  }
Verify the hashes of all pieces in the store and update the bitfield for any new valid
pieces. Useful if data has been added to the store outside WebTorrent, e.g. if another
process puts a valid file in the right place. Once the scan is complete,
`callback(null)` will be called (if provided), unless the torrent was destroyed during
the scan, in which case `callback` will be called with an error.
@param  {callbackWithError=} cb