BUG/MINOR: stream: fix tv_request calculation for applets

When the stats code was moved to an applet, it wasn't completely
cleaned of its usage of the HTTP transaction and it used to store
the HTTP status in txn->status and to set the HTTP request date to
<now> from within the applet. This is totally wrong because the
applet is seen as a server from the HTTP engine, which parses its
response, so the http_txn must not be touched there.

This was made visible by the cache which would always exhibit a
negative TR log, indicating that nowhere in the code we took care of
setting s->logs.tv_request while the code above used to continue to
hide this. Another side effect of this issue is that under load, if
the stats applet call risks to be delayed, the reported t_queue can
appear negative by being below tv_request-tv_accept.

This patch removes the assignment of tv_request and txn->status from
the applet code and instead sets the tv_request if still unset when
connecting to the applet. This ensures that all applets report correct
request timers now.
diff --git a/src/stream.c b/src/stream.c
index 25be1e8..d98c7f9 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -1072,6 +1072,8 @@
 			return;
 		}
 
+		if (tv_iszero(&s->logs.tv_request))
+			s->logs.tv_request = now;
 		s->logs.t_queue   = tv_ms_elapsed(&s->logs.tv_accept, &now);
 		si->state         = SI_ST_EST;
 		si->err_type      = SI_ET_NONE;