timers: fix unref() memory leak

The destructor isn't being called for timers that have been unref'd.

Fixes: nodejs/node-v0.x-archive#8364
PR-URL: #1330
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/timers.js

@@ -301,6 +301,14 @@ const Timeout = function(after) {
   this._repeat = null;
 };
 
+
+function unrefdHandle() {
+  this.owner._onTimeout();
+  if (!this.owner.repeat)
+    this.owner.close();
+}
+
+
 Timeout.prototype.unref = function() {
   if (this._handle) {
     this._handle.unref();
@@ -315,7 +323,8 @@ Timeout.prototype.unref = function() {
     if (this._called && !this._repeat) return;
 
     this._handle = new Timer();
-    this._handle[kOnTimeout] = this._onTimeout;
+    this._handle.owner = this;
+    this._handle[kOnTimeout] = unrefdHandle;
     this._handle.start(delay, 0);
     this._handle.domain = this.domain;
     this._handle.unref();