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/request.go

@@ -571,9 +571,12 @@ func (rq *request) responseReceived() {
 	rq.stopTimers()
 }
 
-// setResponseDeadline sends the status and headers of the app's answer, with
-// at least minSendTime, before ReverseProxy reads the answer's body, and then
-// the rest has until SWRP_MAX_RESPONSE_TIME_MS after the end of the request.
+// setResponseDeadline sets SWRP_MAX_RESPONSE_TIME_MS on the connection to the
+// visitor once ReverseProxy has written the status of the app's answer: the
+// answer must reach the visitor within it of the end of the request. First it
+// sends the status and headers, with minSendTime at least, before ReverseProxy
+// reads the body: the time running out can still cut the answer off, but the
+// visitor never gets a closed connection with no answer at all.
 func (rq *request) setResponseDeadline() {
 	limit := rq.h.config.MaxResponseTime
 	if limit == 0 {