_markUnverified (index) {
    const len = (index === this.pieces.length - 1)
      ? this.lastPieceLength
      : this.pieceLength
    this.pieces[index] = new Piece(len)
    this.bitfield.set(index, false)
    if (!this._startAsDeselected) this.select(index, index, 1)
    for (const file of this.files) {
      if (file.done && file.includes(index)) file.done = false
    }
  }
Mark one piece as unverified
@param {number} index
@private