Upload stalled: a clear answer in 30 seconds, not a 503 after two minutes

A request whose body stops arriving for 30 seconds, such as a chunk of a large upload, is now answered at once with the bytes received, where before it waited about two minutes and answered 503 api_unavailable. A body that keeps arriving, however slowly, is never cut. The SDKs and the CLI get 503 body_stalled and send it again by themselves with the same key; other clients get 408 request_timeout. See errors.