_addIncomingPeer (peer) {
    if (this.destroyed) return peer.destroy(new Error('torrent is destroyed'))
    if (this.paused) return peer.destroy(new Error('torrent is paused'))

    this._debug('add incoming peer %s', peer.id)

    this._registerPeer(peer)
  }
Called whenever a new incoming TCP peer connects to this torrent swarm. Called with a
peer that has already sent a handshake.