setTimeout: do not calculate Timeout._when property

Dramatically improves Timer performance.

lib/timers.js

@@ -263,12 +263,15 @@ var Timeout = function(after) {
   this._idleTimeout = after;
   this._idlePrev = this;
   this._idleNext = this;
-  this._when = Date.now() + after;
+  this._idleStart = null;
+  this._onTimeout = null;
 };
 
 Timeout.prototype.unref = function() {
   if (!this._handle) {
-    var delay = this._when - Date.now();
+    var now = Date.now();
+    if (!this._idleStart) this._idleStart = now;
+    var delay = this._idleStart + this._idleTimeout - now;
     if (delay < 0) delay = 0;
     exports.unenroll(this);
     this._handle = new Timer();