Moved functionality from HostingEnvironment into IUmbracoApplicationLifetime, to not have duplicated logic

src/Umbraco.Core/Net/IUmbracoApplicationLifetime.cs

@@ -2,7 +2,13 @@ namespace Umbraco.Net
 {
     public interface IUmbracoApplicationLifetime
     {
+        /// <summary>
+        /// A value indicating whether the application is restarting after the current request.
+        /// </summary>
         bool IsRestarting { get; }
+        /// <summary>
+        /// Terminates the current application. The application restarts the next time a request is received for it.
+        /// </summary>
         void Restart();
     }
 }