rproxy: leave no visitor without an answer as the response time runs out (closes #295)

A visitor could get a closed connection and no answer when
SWRP_MAX_RESPONSE_TIME_MS ran out about when the answer started: the
write deadline passed before the first byte was written.

- An answer rproxy gives itself has at least 10 seconds to be sent.
- An app's answer that arrives once the time has run out is answered
  504.
- Otherwise the app's status and headers are sent at once, before the
  body is read, with at least 10 seconds. The exact limit applies to
  the rest.

Model: opus-5-5

internal/rproxy/bodies.go

@@ -163,6 +163,8 @@ type responseWriter struct {
 	// status is the final status sent, or zero before one is.
 	status int
 	bytes  int64
+	// afterStatus, if set, is called once the final status has been written.
+	afterStatus func()
 }
 
 // WriteHeader sends the status and headers. An informational 1xx status is
@@ -173,6 +175,10 @@ func (w *responseWriter) WriteHeader(status int) {
 	}
 
 	w.ResponseWriter.WriteHeader(status)
+
+	if status >= http.StatusOK && w.afterStatus != nil {
+		w.afterStatus()
+	}
 }
 
 // Write sends part of the body.