_verifyPieces (cb) {
    if (this._hasStartupBitfield) {
      return this._verifyPiecesUsingBitfield(cb)
    }
    this._verifyPiecesUsingHash(this.pieces, cb)
  }
Verifies the pieces of the torrent, if a startup bitfield is provided it will be used for verification
@param {callbackWithError} cb
@private