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

docs/settings.md

@@ -436,8 +436,9 @@ A time here is a whole number of milliseconds above zero, such as `2000`.
   headers, the time until the app has been sent the whole request. A time or
   `off`.
 - `SWRP_MAX_RESPONSE_TIME_MS` (default `1800000`): how long the answer may take,
-  from the end of the request to its last byte reaching the visitor. A time or
-  `off`.
+  from the end of the request to its last byte reaching the visitor. An answer
+  `rproxy` gives itself always has at least 10 seconds to be sent, and so do the
+  status and headers of the app's answer. A time or `off`.
 - `SWRP_LISTEN_ADDR` (default `:8080`): where `rproxy` listens, such as `:9000`.
 - `SWRP_UPSTREAM_URL` (default `http://127.0.0.1:8081`): the app. An `http` or
   `https` URL with a host and an optional port, and nothing more.