Missing handlers for torrent events

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Background

The handlers for the following alerts are missing

• torrent_checked_alert
• read_piece_alert
• piece_finished_alert

see

  _torrentCheckedAlert (alert) {
  }

  _readPieceAlert (alert) {
  }

  _pieceFinishedAlert (alert) {
  }

Problem

Are needed for various purposes, in particular for the torrent state machine and the live streaming.

Solution

Implement handlers that emit a corresponding signal on the torrent wrapper.