torrent._update performance improvement

Queue torrent updates once per second, as it is an expensive operation. Before, this would cause updates to happen nearly every tick.

lib/torrent.js

@@ -1171,14 +1171,23 @@ class Torrent extends EventEmitter {
    * Heartbeat to update all peers and their requests.
    */
   _update () {
-    if (this.destroyed) return
-
-    // update wires in random order for better request distribution
-    const ite = randomIterate(this.wires)
-    let wire
-    while ((wire = ite())) {
-      this._updateWireWrapper(wire)
+    if (this._updateScheduled) {
+      this._debug('_update already queued')
+      return
     }
+
+    this._updateScheduled = true
+    setTimeout(() => {
+      this._updateScheduled = false
+      if (this.destroyed) return
+
+      // update wires in random order for better request distribution
+      const ite = randomIterate(this.wires)
+      let wire
+      while ((wire = ite())) {
+        this._updateWireWrapper(wire)
+      }
+    }, 1000)
   }
 
   _updateWireWrapper (wire) {