Extended the server library to include properties Started and LastRestart in its application state representation (class AppInfo).
Changed AppInfo.Started and .Restarted from DateTime to Nullable<DateTime> to be able to distinguish applications that have never started/restarted, from those that have.
Changed the admin server library to assign the application state "Started" property when an application is started.
Extended the admin server library to assign the last restart time on applications that are being restarted because a fellow application in the same code host needs to stop (or restart).
Made sure the admin server ExecutableHandler set the proper start- and restart time on application entities it hands out, based on the server library state holding such information.
Merge branch 'issue/1646' into develop. Fixes #1646.