Abandon timer coroutine handles during shutdown to avoid stack overflow

Same pattern as the IOCP shutdown fix (a397ea4): replace h_.destroy()
with h_ = {} to abandon the coroutine handle instead of destroying it.
Destroying timer waiter coroutines during shutdown can trigger re-entrant
destruction through circular frame ownership.

Author: sgerbino

include/boost/corosio/detail/timer_service.hpp

@@ -335,7 +335,7 @@ timer_service::shutdown()
         while (auto* w = impl->waiters_.pop_front())
         {
             w->stop_cb_.reset();
-            w->h_.destroy();
+            w->h_ = {};
             sched_->work_finished();
             delete w;
         }