Simplified away CONNECT::real_id

It was wrong anyway, as it may not get updated by the time created thread
reads it. And it was 0 for cached threads.

Part of MDEV-19515 - Improve connect speed

sql/mysqld.cc

@@ -6208,8 +6208,9 @@ void create_thread_to_handle_connection(CONNECT *connect)
   DBUG_PRINT("info",(("creating thread %lu"), (ulong) connect->thread_id));
   connect->prior_thr_create_utime= microsecond_interval_timer();
 
+  pthread_t tmp;
   if (auto error= mysql_thread_create(key_thread_one_connection,
-                                      &connect->real_id, &connection_attrib,
+                                      &tmp, &connection_attrib,
                                       handle_one_connection, (void*) connect))
   {
     char error_message_buff[MYSQL_ERRMSG_SIZE];

sql/sql_connect.cc

@@ -1522,6 +1522,6 @@ THD *CONNECT::create_thd(THD *thd)
                            my_localhost : 0;
 
   thd->scheduler=          scheduler;
-  thd->real_id=            real_id;
+  thd->real_id= pthread_self(); /* Duplicates THD::store_globals() setting. */
   DBUG_RETURN(thd);
 }

sql/sql_connect.h

@@ -34,14 +34,13 @@ class CONNECT : public ilink {
   Vio *vio;                           /* Copied to THD with my_net_init() */
   scheduler_functions *scheduler;
   my_thread_id thread_id;
-  pthread_t    real_id;
 
   /* Own variables */
   bool thread_count_incremented;
   ulonglong    prior_thr_create_utime;
 
   CONNECT()
-    :vio(0), scheduler(thread_scheduler), thread_id(0), real_id(0),
+    :vio(0), scheduler(thread_scheduler), thread_id(0),
     thread_count_incremented(0), prior_thr_create_utime(0)
   {
   };