[http server]: use tokio::spawn internally in `HttpServer::start` and return `StopHandle` (#402)

* [ws server]: refactor start()

* remove needless clone

* fix http server too

* fmt

* unify stop APIs

* fix bad merge

* add API to use custom tokio runtime

* use futures oneshot

* fix unintentional change

* Revert "use futures oneshot"

This reverts commit 0432f6ba41d49a3494943458657a31f8afd79160.

* Update http-server/src/server.rs

Co-authored-by: Maciej Hirsz <[email protected]>

Co-authored-by: Maciej Hirsz <[email protected]>