rproxy: cut off an answer that stalls after arriving before the whole request (closes #345)

When the app's answer arrived before the app had been sent the whole
request, SWRP_MAX_RESPONSE_TIME_MS had no timer: only the end of the
request started one. If the app then stalled, only a write could fail at
the deadline. With nothing written, the connection stayed open until the
request time limit.

modifyResponse now starts the response timer when it ends the request.
Once the timer cuts off the app's answer, nothing is left to flush, so the
last flush in finish succeeded and Go's server read the rest of the body.
finish now also ends that reading at once after a refusal of an answer
already passed on.

Model: opus-5-5

internal/rproxy/request.go

@@ -581,7 +581,8 @@ func (rq *request) startResponseTimer(requestEnd time.Time) {
 		return
 	}
 
-	rq.responseTimer = time.AfterFunc(time.Until(requestEnd.Add(limit)), rq.responseTimedOut)
+	rq.responseTimer = time.AfterFunc(time.Until(requestEnd.Add(limit)),
+		rq.responseTimedOut)
 }
 
 // responseTimedOut is called when SWRP_MAX_RESPONSE_TIME_MS runs out before the