rproxy: send its own answers at once to a visitor that stops sending its body (closes #303)

An answer rproxy gives itself, such as a deny or a refusal, waited while
Go's server read up to 256 KiB of a body that had not arrived whole. A
visitor that stopped sending got nothing until SWRP_MAX_REQUEST_TIME_MS
ran out, and no bytes at all once that was past the 10 seconds the
answer has to be sent.

send now sets Connection: close in that case, as modifyResponse does for
the app's answer; both call closeAfterAnswer. requestTimedOut no longer
ends that read of Go's server, which no longer happens, so the time the
app's answer started is no longer noted.

Model: opus-5-5

internal/rproxy/request.go

@@ -83,8 +83,8 @@ type request struct {
 	upgraded *upgradedConn
 
 	// mu guards what follows. The time limits run on goroutines of their own,
-	// and the transport starts and stops them, and notes the times below, from
-	// its own; once timersStopped is set, neither time limit acts any more.
+	// and the transport starts and stops them, and notes requestEnd, from its
+	// own; once timersStopped is set, neither time limit acts any more.
 	mu            sync.Mutex
 	timersStopped bool
 	requestTimer  *time.Timer