Ensure the EventWaitHandle is cleaned up, MainDom and BackgroundTaskRunner was not using HostingEnvironment.Stop correctly, ensures no exception is thrown when setting task result on a background task, ensure multiple terminations don't occur, ensure terminations are done if shutdown fails.

src/Umbraco.Core/SystemLock.cs

@@ -120,6 +120,8 @@ namespace Umbraco.Core
                     {
                         _semaphore.Release();
                         _semaphore.Dispose();
+
+                        
                     }
 
                     // free unmanaged resources (unmanaged objects) and override a finalizer below.