MDEV-6150 Speed up connection speed by moving creation of THD to new thread

Creating a CONNECT object on client connect and pass this to the working thread which creates the THD.
Split LOCK_thread_count to different mutexes
Added LOCK_thread_start to syncronize threads
Moved most usage of LOCK_thread_count to dedicated functions
Use next_thread_id() instead of thread_id++

Other things:
- Thread id now starts from 1 instead of 2
- Added cast for thread_id as thread id is now of type my_thread_id
- Made THD->host const (To ensure it's not changed)
- Removed some DBUG_PRINT() about entering/exiting mutex as these was already logged by mutex code
- Fixed that aborted_connects and connection_errors_internal are counted in all cases
- Don't take locks for current_linfo when we set it (not needed as it was 0 before)

sql/threadpool.h

@@ -38,10 +38,12 @@ extern int  threadpool_add_connection(THD *thd);
   threadpool_unix.cc or threadpool_win.cc
 */
 extern bool tp_init();
-extern void tp_add_connection(THD*);
+extern bool tp_init_new_connection_thread();
+extern void tp_add_connection(CONNECT *);
 extern void tp_wait_begin(THD *, int);
 extern void tp_wait_end(THD*);
 extern void tp_post_kill_notification(THD *thd);
+extern bool tp_end_thread(THD *thd, bool cache_thread);
 extern void tp_end(void);
 
 /* Used in SHOW for threadpool_idle_thread_count */