`HttpServer::new` should support asynchronous application factories

## Expected Behavior
`HttpServer::new` should support asynchronous application factories as the argument.

## Current Behavior
```rust
HttpServer::new(move || async move { ... });
```
fails.

## Possible Solution
We either add `new_async` method or somehow make `new` work for both sync and async (if possible).

## Context
I tried to allocate a thread object whose allocator uses an async method. It failed and now I don't know what to do.

## Your Environment

- Rust Version: 1.58.1
- Actix Web Version: 4.0.0-rc.1