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/scheduler.h

@@ -37,7 +37,7 @@ struct scheduler_functions
   ulong *max_connections;
   bool (*init)(void);
   bool (*init_new_connection_thread)(void);
-  void (*add_connection)(THD *thd);
+  void (*add_connection)(CONNECT *connect);
   void (*thd_wait_begin)(THD *thd, int wait_type);
   void (*thd_wait_end)(THD *thd);
   void (*post_kill_notification)(THD *thd);