timers: add assertion

Issue #178, and recent netperf 3 experiments by Vlad, suggests we have a
bug in our timer code which very rarely causes crashes on
timer_list::fired(). It appears we somehow corrupt our per-cpu timer list,
or some armed timer object - but I still haven't been able to figure out
where.

This patch adds an assertion that the timer we find on the list is
actually armed. Unfortunately, it consistantly fails (I can see the same
assertion failure once every 100-200 runs of tst-queue-mpsc.so), and this
failure seems to replace the page-fault crash of issue #178.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

core/sched.cc

@@ -986,6 +986,7 @@ void timer_list::fired()
     // don't hold iterators across list iteration, since the list can change
     while (!_list.empty() && _list.begin()->_time <= now) {
         auto j = _list.begin();
+        assert(j->_state == timer_base::state::armed);
         _list.erase(j);
         j->expire();
     }