BUG/MEDIUM: proxy: use thread-safe stream killing on hard-stop

BUG/MEDIUM: proxy: use thread-safe stream killing on hard-stop

When setting hard-stop-after, hard_stop() is called at the end to kill
last pending streams. Unfortunately there's no locking there while
walking over the streams list nor when shutting them down, so it's
very likely that some old processes have been crashing or gone wild
due to this. Let's use a thread_isolate() call for this as we don't
have much other choice (and it happens once in the process' life,
that's OK).

This must be backported to 1.8.