BUG/MAJOR: listener: do not schedule a task-less proxy

Apparently seamingless commit 0591bf7deb ("MINOR: listener: make the
wait paths cleaner and more reliable") caused a nasty regression and
revealed a rare race that hits regtest stickiness/lb-services.vtc
about 4% of the times for 8 threads.

The problem is that when a multi-threaded listener wakes up on an
incoming connection, several threads can receive the event, especially
when idle. And all of them will race to accept the connections in
parallel, adjusting the listener's nbconn and proxy's feconn until
one reaches the proxy's limit and declines. At this step the changes
are cancelled, the listener is marked "limited", and when the threads
exit the function, one of them will unlimit the listener/proxy again
so that it can accept incoming connections again.

The problem happens when many threads connect to a small peers section
because its maxconn is very limited (typically 6 for 2 peers), and it's
sometimes possible for enough competing threads to hit the limit and
one of them will limit the listener and queue the proxy's task... except
that peers do not initialize their proxy task since they do not use rate
limiting. Thus the process crashes when doing task_schedule(p->task).
Prior to the cleanup patch above, this didn't happen because the error
path that was dedicated to only limiting the listener did not call
task_schedule(p->task).

Given that the proxy's task is optional, and that the expire value
passed there is always TICK_ETERNITY, it's sufficient and reasonable to
avoid calling this task_schedule() when expire is not set. And for long
term safety we can also avoid to do it when the task is not set. A first
fix consisted in allocating a task for the peers proxies but it's never
used and would eat resources for reason.

No backport is needed as this commit was only merged into 2.2.

src/listener.c

@@ -1102,7 +1102,8 @@ void listener_accept(int fd)
 	 * later than <expire> ahead. The listener turns to LI_LIMITED.
 	 */
 	limit_listener(l, &p->listener_queue);
-	task_schedule(p->task, expire);
+	if (p->task && tick_isset(expire))
+		task_schedule(p->task, expire);
 	goto end;
 }