Try to reduce dl impact on slow computers (web)

[Chocobozzz]

On slow computers with a fast network, web browsers may no longer respond because webtorrent is downloading chunks too fast.

The main issue is that you cannot stream a video torrent: the web browser does not have time to play the video, it is too busy downloading the video chunks.

On web browser supporting requestIdleCallback, we download chunks with a lower priority allowing the web browser to execute other tasks (like playing the video).

[DiegoRBaquero]

I like this. LGTM

[ThaUnknown]

this kind of annoys me, because it's a REALLY good change, that benefits slow computers, but on the other hand it's one of the heaviest functions to run, so for fast computers some 20% of the CPU time goes to scheduling this

[ThaUnknown]

the old code for this was:

    const ite = randomIterate(this.wires)
    let wire
    while ((wire = ite())) {
      this._updateWireWrapper(wire)
    }
  }

  _updateWireWrapper (wire) {
    const self = this

    if (typeof window !== 'undefined' && typeof window.requestIdleCallback === 'function') {
      window.requestIdleCallback(function () { self._updateWire(wire) })
    } else {
      self._updateWire(wire)
    }
  }

I'm suggesting we flip it, that requestIdleCallback is called before random iterate, then after the callback all the peers are updated, this should still help performance while not dwindling ultra-high data transfer speeds:

  /**
   * Heartbeat to update all peers and their requests.
   */
  _update () {
    if (IDLE_CALLBACK) {
      IDLE_CALLBACK(() => this._updateWireWrapper(), { timeout: 250 })
    } else {
      this._updateWireWrapper()
    }
  }

  _updateWireWrapper () {
    if (this.destroyed) return
    // update wires in random order for better request distribution
    const ite = randomIterate(this.wires)
    let wire
    while ((wire = ite())) {
      this._updateWire(wire)
    }
  }

@Chocobozzz do you think this will still accomplish its original goal? There aren't really many reproduction steps provided, and I haven't ran into the issue you're describing with this disabled, so this is kind of a blind change