CookOnTheFly: No message when starting creation of asset registry

UCookOnTheFlyServer::GenerateAssetRegistry() logs when the registry has been created and how long it took, but there is no message to indicate when the operation started (unless using DEBUG_COOKONTHEFLY). This operation could potentially take a long time, so we shoudl indicate to the user that the operation is in progress.

Create vanilla project
Launch on WindowsNoEditor using default settings
Wait for project to launch
Look for "Done creating registry" in log
Note that there was no preceding message indicating that the registry was being created