Move all kill mutex protection to LOCK_thd_kill

LOCK_thd_data was used to protect both THD data and
ensure that the THD is not deleted while it was in use

This patch moves the THD delete protection to LOCK_thd_kill,
which already protects the THD for kill.

The benefits are:
- More well defined what LOCK_thd_data protects
- LOCK_thd_data usage is now much simpler and easier to verify
- Less chance of deadlocks in SHOW PROCESS LIST as there is less
  chance of interactions between mutexes
- Remove not needed LOCK_thread_count from
  thd_get_error_context_description()
- Fewer mutex taken for thd->awake()

Other things:
- Don't take mysys->var mutex in show processlist to check if thread
  is kill marked
- thd->awake() now automatically takes the LOCK_thd_kill mutex
  (Simplifies code)
- Apc uses LOCK_thd_kill instead of LOCK_thd_data

sql/event_scheduler.cc

@@ -648,14 +648,11 @@ Event_scheduler::stop()
     state= STOPPING;
     DBUG_PRINT("info", ("Scheduler thread has id %lu",
                         (ulong) scheduler_thd->thread_id));
-    /* Lock from delete */
-    mysql_mutex_lock(&scheduler_thd->LOCK_thd_data);
     /* This will wake up the thread if it waits on Queue's conditional */
     sql_print_information("Event Scheduler: Killing the scheduler thread, "
                           "thread id %lu",
                           (ulong) scheduler_thd->thread_id);
     scheduler_thd->awake(KILL_CONNECTION);
-    mysql_mutex_unlock(&scheduler_thd->LOCK_thd_data);
 
     /* thd could be 0x0, when shutting down */
     sql_print_information("Event Scheduler: "