async `HttpServer::new` as alternative to `data_factory`

Part of the apeal of `App::data_factory` is that it uses the local reactor when instantiating app data. It's equivalent and more natural to make `HttpServer::new` async.

Start by comparing the existing App::data_factory behavior with HttpServer::new, focusing on how each uses the local reactor when instantiating app data. Define the async API's intended behavior and compatibility before changing anything; done means HttpServer::new provides the proposed async alternative without losing the data_factory behavior.

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