Try to reduce dl impact on slow computers (web)

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 torrent video: the web browser does not have time to play the video, it is too busy downloading the video chunks.

On web browser supporting [requestIdleCallback](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback#See_also), we download chunks with a lower priority allowing the web browser to execute other tasks (like playing the video).

lib/torrent.js

@@ -1156,7 +1156,11 @@ class Torrent extends EventEmitter {
     if (wire.requests.length >= minOutstandingRequests) return
     const maxOutstandingRequests = getBlockPipelineLength(wire, PIPELINE_MAX_DURATION)
 
-    trySelectWire(false) || trySelectWire(true)
+    if (typeof window !== 'undefined' && typeof window.requestIdleCallback === 'function') {
+      window.requestIdleCallback(function () { trySelectWire(false) || trySelectWire(true) })
+    } else {
+      trySelectWire(false) || trySelectWire(true)
+    }
 
     function genPieceFilterFunc (start, end, tried, rank) {
       return i => i >= start && i <= end && !(i in tried) && wire.peerPieces.get(i) && (!rank || rank(i))