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)

mysql-test/r/log_tables.result

@@ -436,9 +436,9 @@ My own slow query	sleep(2)
 My own slow query	0
 SELECT * FROM mysql.slow_log WHERE seq >= 2 LIMIT 3;
 start_time	user_host	query_time	lock_time	rows_sent	rows_examined	db	last_insert_id	insert_id	server_id	sql_text	thread_id	rows_affected	seq
-START_TIME	USER_HOST	QUERY_TIME	00:00:00.000000	1	0	test	0	0	1	SELECT "My own slow query", sleep(2)	3	0	2
-START_TIME	USER_HOST	QUERY_TIME	00:00:00.000000	1	0	test	0	0	1	SELECT "My own slow query", sleep(2)	3	0	3
-START_TIME	USER_HOST	QUERY_TIME	00:00:00.000000	1	0	test	0	0	1	SELECT "My own slow query", sleep(2)	3	0	4
+START_TIME	USER_HOST	QUERY_TIME	00:00:00.000000	1	0	test	0	0	1	SELECT "My own slow query", sleep(2)	2	0	2
+START_TIME	USER_HOST	QUERY_TIME	00:00:00.000000	1	0	test	0	0	1	SELECT "My own slow query", sleep(2)	2	0	3
+START_TIME	USER_HOST	QUERY_TIME	00:00:00.000000	1	0	test	0	0	1	SELECT "My own slow query", sleep(2)	2	0	4
 SET GLOBAL slow_query_log = 0;
 SET SESSION long_query_time =@saved_long_query_time;
 FLUSH LOGS;