adds appSetting to switch maindom lock

src/Umbraco.Core/Constants-AppSettings.cs

@@ -9,6 +9,8 @@ namespace Umbraco.Core
         /// </summary>
         public static class AppSettings
         {
+            public const string MainDomLock = "Umbraco.Core.MainDom.Lock";
+
             // TODO: Kill me - still used in Umbraco.Core.IO.SystemFiles:27
             [Obsolete("We need to kill this appsetting as we do not use XML content cache umbraco.config anymore due to NuCache")]
             public const string ContentXML = "Umbraco.Core.ContentXML"; //umbracoContentXML

src/Umbraco.Web/UmbracoApplication.cs

@@ -1,7 +1,9 @@
-using System.Threading;
+using System.Configuration;
+using System.Threading;
 using System.Web;
 using Umbraco.Core;
 using Umbraco.Core.Logging.Serilog;
+using Umbraco.Core.Runtime;
 using Umbraco.Web.Runtime;
 
 namespace Umbraco.Web
@@ -14,7 +16,17 @@ namespace Umbraco.Web
         protected override IRuntime GetRuntime()
         {
             var logger = SerilogLogger.CreateWithDefaultConfiguration();
-            return new WebRuntime(this, logger, new MainDom(logger));
+
+            // Determine if we should use the sql main dom or the default
+            var appSettingMainDomLock = ConfigurationManager.AppSettings[Constants.AppSettings.MainDomLock];
+
+            var mainDomLock = appSettingMainDomLock == "SqlMainDomLock"
+                ? (IMainDomLock)new SqlMainDomLock(logger)
+                : new MainDomSemaphoreLock();
+            
+            var runtime = new WebRuntime(this, logger, new MainDom(logger, mainDomLock));
+
+            return runtime;
         }
 
         /// <summary>