Reuse THD for new user connections

- To ensure that mallocs are marked for the correct THD, even if it's
  allocated in another thread, I added the thread_id to the THD constructor
- Added st_my_thread_var to thr_lock_info_init() to avoid a call to my_thread_var
- Moved things from THD::THD() to THD::init()
- Moved some things to THD::cleanup()
- Added THD::free_connection() and THD::reset_for_reuse()
- Added THD to CONNECT::create_thd()
- Added THD::thread_dbug_id and st_my_thread_var->dbug_id. These are needed
  to ensure that we have a constant thread_id used for debugging with a THD,
  even if it changes thread_id (=connection_id)
- Set variables.pseudo_thread_id in constructor. Removed not needed sets.

sql/sql_class.h

@@ -2652,7 +2652,7 @@ public:
   ulong      query_plan_flags; 
   ulong      query_plan_fsort_passes; 
   pthread_t  real_id;                           /* For debugging */
-  my_thread_id  thread_id;
+  my_thread_id  thread_id, thread_dbug_id;
   uint32      os_thread_id;
   uint	     tmp_table, global_disable_checkpoint;
   uint	     server_status,open_options;
@@ -2764,7 +2764,7 @@ public:
   /* for IS NULL => = last_insert_id() fix in remove_eq_conds() */
   bool       substitute_null_with_insert_id;
   bool	     in_lock_tables;
-  bool       bootstrap, cleanup_done;
+  bool       bootstrap, cleanup_done, free_connection_done;
 
   /**  is set if some thread specific value(s) used in a statement. */
   bool       thread_specific_used;
@@ -2906,7 +2906,7 @@ public:
   /* Debug Sync facility. See debug_sync.cc. */
   struct st_debug_sync_control *debug_sync_control;
 #endif /* defined(ENABLED_DEBUG_SYNC) */
-  THD(bool is_wsrep_applier= false);
+  THD(my_thread_id id, bool is_wsrep_applier= false);
 
   ~THD();
 
@@ -2926,6 +2926,8 @@ public:
   void change_user(void);
   void cleanup(void);
   void cleanup_after_query();
+  void free_connection();
+  void reset_for_reuse();
   bool store_globals();
   void reset_globals();
 #ifdef SIGNAL_WITH_VIO_CLOSE