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/sql_class.h

@@ -2127,11 +2127,15 @@ public:
     - thd->query and thd->query_length (used by SHOW ENGINE
       INNODB STATUS and SHOW PROCESSLIST
     - thd->db and thd->db_length (used in SHOW PROCESSLIST)
-    - thd->mysys_var (used by KILL statement and shutdown).
     Is locked when THD is deleted.
   */
   mysql_mutex_t LOCK_thd_data;
-  /* Protect kill information */
+  /*
+    Protects:
+    - kill information
+    - mysys_var (used by KILL statement and shutdown).
+    - Also ensures that THD is not deleted while mutex is hold
+  */
   mysql_mutex_t LOCK_thd_kill;
 
   /* all prepared statements and cursors of this connection */
@@ -3131,7 +3135,13 @@ public:
   }
   void close_active_vio();
 #endif
-  void awake(killed_state state_to_set);
+  void awake_no_mutex(killed_state state_to_set);
+  void awake(killed_state state_to_set)
+  {
+    mysql_mutex_lock(&LOCK_thd_kill);
+    awake_no_mutex(state_to_set);
+    mysql_mutex_unlock(&LOCK_thd_kill);
+  }
  
   /** Disconnect the associated communication endpoint. */
   void disconnect();