HTTP server to support request body read timeout

This could be done manually by switching to `raw-handler` and tracking time we need to read chunks from the stream, but in most general use case like

```clojure
(defn handler [{:keys [body]}]
(json/parse-string (bs/to-string body)) true)

(http/start-server handler {:port port})
```

a client (either malicious or buggy) could easily consume a thread per each connection by sending a request with a body that exceeds `:request-buffer-size` slowly enough (or just pausing forever). In such a case we invoke `handler` on a `response-executor` before the body is read, meaning that call to `bs/to-string` effectively blocks a thread allocated.

Start by tracing the HTTP server handling around raw-handler, response-executor, and :request-buffer-size. Clarify the timeout behavior and configuration expected for slow or paused request bodies; done means such requests no longer occupy response-executor threads while normal body parsing continues.

Written by the indexing model from the issue text.