refactor Scheduler to use new BackgroundTaskRunner capabilities

Conflicts:
	src/Umbraco.Web/Scheduling/Scheduler.cs

src/Umbraco.Web/Scheduling/DelayedRecurringTaskBase.cs

@@ -34,12 +34,18 @@ namespace Umbraco.Web.Scheduling
 
                 if (_gate != null) return _gate;
                 _gate = new ManualResetEvent(false);
+
+                // note
+                // must use the single-parameter constructor on Timer to avoid it from being GC'd
+                // read http://stackoverflow.com/questions/4962172/why-does-a-system-timers-timer-survive-gc-but-not-system-threading-timer
+
                 _timer = new Timer(_ =>
                 {
                     _timer.Dispose();
                     _timer = null;
                     _gate.Set();
-                }, null, _delayMilliseconds, 0);
+                });
+                _timer.Change(_delayMilliseconds, 0);
                 return _gate;
             }
         }