MDEV-37728: Fix shutdown deadlock between thr_timer_end and set_period

The mysys timer (thr_timer) could deadlock with
srv_thread_pool_end() due to lock inversion:

- LOCK_timer is held during the entire timer action routine.
- timer action executed in mysys thread may lock timer_generic::m_mtx.
- main thread  holding the same timer_generic::m_mtx calls
  thr_timer_end(), which also needs LOCK_timer.

This inversion can cause a very rare shutdown deadlock, when the
threadpool extends the maintenance() period after idling for a
minute and shutdown occurs simultaneously.

Workaround: Use try_lock in timer_generic::set_period() to avoid blocking,
to break the deadlock cycle, and ignore errors.

Root cause is the thr_timer holds the global LOCK_timer for the
entire duration of every timer action. This makes it difficult or
impossible to use additional locks safely inside timer routines.
Fortunately, in this case a failed try_lock has no impact.

Author: vaintroub

tpool/tpool_generic.cc

@@ -393,7 +393,9 @@ class thread_pool_generic : public thread_pool
     */
     void set_period(int period_ms)
     {
-      std::unique_lock<std::mutex> lk(m_mtx);
+      std::unique_lock<std::mutex> lk(m_mtx, std::defer_lock);
+      if (!lk.try_lock())
+        return;
       if (!m_on)
         return;
       if (!m_pool)