Ensure MVC Controller and dependencies are fully asynchronous

[JeremyCaney]

As part of the lazy-loading project (#111), I made all of the ITopicRepository members async. There remain a couple of actions in the OnTopic.AspNetCore library that are synchronous, however, and thus can't take advantage of this—not to mention other performance features of ASP.NET Core. There are also a couple of dependencies that are still synchronous, such as methods on the IHierarchicalTopicRepository.

• Convert RedirectController to async
• Convert SitemapController to async
• Convert IHierarchicalTopicRepository.GetHierarchicalRoot() to async
• Convert Load(topicRepository, routeData) to async (in TopicRepositoryExtensions)
• Convert CurrentTopic to async GetCurrentTopic() (in TopicController, ErrorController)
• Rename IndexAsync() and HttpAsync() to Index() and Http() (in ErrorController)
• Replace base.OnActionExecuting(context) with await next().ConfigureAwait(false) (in ValidateTopicAttribute, TopicResponseCacheAttribute
• Create unit tests to validate the above logic