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/opt_subselect.cc

@@ -3944,7 +3944,7 @@ SJ_TMP_TABLE::create_sj_weedout_tmp_table(THD *thd)
   {
     /* if we run out of slots or we are not using tempool */
     sprintf(path,"%s%lx_%lx_%x", tmp_file_prefix,current_pid,
-            thd->thread_id, thd->tmp_table++);
+            (ulong) thd->thread_id, thd->tmp_table++);
   }
   fn_format(path, path, mysql_tmpdir, "", MY_REPLACE_EXT|MY_UNPACK_FILENAME);