add HttpServer::new_async

[aliemjay]

PR Type

Feature

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

Closes #2286

[aliemjay]

Depends on actix/actix-net#397

FnOnce was chosen for the closure because otherwise it would be incovenient to use - the users are required to clone captures before the async block:

HttpServer::new_async(move || {
    let data = data.clone();
    async move { App::new().data(data) }
})

HttpServer::new was relaxed to accept FnOnce too for consistency.

[robjtede]

not pursuing async server builders this cycle so delaying this til post v4