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/sql_test.cc

@@ -88,7 +88,7 @@ static my_bool print_cached_tables_callback(TDC_element *element,
     THD *in_use= entry->in_use;
     printf("%-14.14s %-32s%6ld%8ld%6d  %s\n",
            entry->s->db.str, entry->s->table_name.str, element->version,
-           in_use ? in_use->thread_id : 0,
+           in_use ? (long) in_use->thread_id : (long) 0,
            entry->db_stat ? 1 : 0,
            in_use ? lock_descriptions[(int)entry->reginfo.lock_type] :
                     "Not in use");