Add HttpTaskAsyncHandler implementation

The THttpHandler doesn't support an async pipeline. This means that it's difficult for service implementations to make async calls. If there was an implementation of HttpTaskAsyncHandler, you could write services using async calls.

Additionally, if you generate the C# classes with the current async support, you get a single interface with both sync and async methods. This doesn't really make sense on the server side since if you implement a service you end up leaving all the async method unimplemented. It would be useful if there were separate sync and async interfaces to make this a bit tidier.