cool project.

However, the callback has an odd interface.

why not pass a universal ref T&& vs a const T&

most of your code is like this.

``` auto resp = HttpResponse::newHttpJsonResponse(json); callback(resp); ```

which means callback can reuse the buffer if it wanted to rather than making a copy because it's const ref.

PS: did you benchmark it against Folly/Proxygen (facebook) and Seastar